| Message ID | 20250120212418.889385-1-rrendec@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [RFC] firmware: arm_scmi: Support mailbox transports with no completion IRQ | expand |
On Mon, Jan 20, 2025 at 04:24:18PM -0500, Radu Rendec wrote: > With the introduction of no_completion_irq in struct scmi_chan_info in > commit a690b7e6e774 ("firmware: arm_scmi: Add configurable polling mode > for transports") it became possible to enable polling mode by default > when the transport does not support completion interrupts. > Hi Radu, > Mailbox controllers on the other hand have a similar mechanism to > indicate if completion interrupts are available, using the txdone_irq > flag in struct mbox_controller. This is available since the introduction > of the mailbox framework in commit 2b6d83e2b8b7 ("mailbox: Introduce > framework for mailbox"). > mmm...I dont think you can do this....it is a bit tricky to explain BUT the optional txdone_irq in the mailbox subsystem is NOT a completion interrupt as intended in the SCMI world, I'll try to explain why in the following. The SCMI mailbox transport is defined by a mailbox and an associated shared memory area: when an SCMI client like Linux wants to send a message to the server it places the message in the shmem, set the channel BUSY bit, and rings the doorbell to alert the other side; at this point the SCMI server wakes up grab the message from the shmem, process it as it sees fit, places the reply back into the same shmem area, clears the channnel BUSY bit, and CAN optionally ring back the client with the mailbox doorbell: this completion IRQ is OPTIONAL, and, if missing, the client will have instead to revert to polling the well defined channel BUSY bit in the shmem area to know when the reply from the server is ready. The end result is that the channel is kept busy until the server has replied, and cannot be reused until we are sure that the reply is available AND that the client execution path, waiting for it, has comsumed such expected reply message. (or times out) Alternatively, even if we have this completion IRQ we can decide, client side, to start a polling trabnsaction and dont use the completion IRQ...but this is another sroty. Now, in the mailbox subsystem the txdone_irq, when present is meant to represnt a txACK IRQ, sent automatically by the mailbox controller, when the transmission has completed and the issued command has been read by the server(usually when the server clears some specific reg): it does NOT assure the client that the server has processed the request and that a reply has been placed into the shmem area, so it is not what SCMI intend of a completion interrupt: it is just a mere indication that the transmission client->server has completed in the mailbox and that the mailbox channel(doorbell) is now free annd available for another transmission. Indeed, such TxACK, when received by the mailbox driver causes the next enqueued message to be sent (i.e. the doorbell to be ring), and, while it is certainly useful in some other context in which you could use such TxACK to deduce that the channel is free and available for another transmission, it is not enough in the SCMI world to guarantee that the SCMI reply has come back and has been processed by the client. From another point of view, you could say that the problem is that the mailbox/doorbell/TxACK is only one side of the story, the other part, the shared memory area used for the effective message transmission, is unknown to the mailbox controller and in control of the SCMI stack, so you cannot let the mailbox subsystem decide when to queue new messages on the same shared memory area used for cmds and replies. In fact, such TxACk interrupt has no practical usage in the SCMI stack and it is even counterproductive to have it, since it can cause the client to mistakenly attempt the next transmission before the previous one has completed, so overwriting the in-flight reply. (..and our busy-looping client side on the channel BUSY bit does not help here...) The following commit, though, needed mainly for different reasons, should have indeed solved also the issue with mailbox controllers having a TxACK, since it introduces a global channel lock that serializes and inhibits any further transmissions, at the SCMI layer, even on systems that has a working and enabled TxACK IRQ. commit da1642bc97c4ef67f347edcd493bd0a52f88777b (tag: scmi-fixes-6.12) Author: Justin Chen <justin.chen@broadcom.com> Date: Mon Oct 14 09:07:17 2024 -0700 firmware: arm_scmi: Queue in scmi layer for mailbox implementation In general, anyway, it would be even better, if possible, not to enable at all such txACK IRQ at the mailbox level when such a controller is used for SCMI: as an example, in arm_mhuv3.c, NOT even defining the txACK in the DT when describimng the mailbox controller does the trick. Did you see any specific anomaly regarding the SCMI stack when using a mailbox controller which provides a txACK ? Sorry for the not so short mail :P ... but if I am missing something and you are seeing anomalies, please let us know. Thanks, Cristian
Hi Cristian, On Tue, 2025-01-21 at 10:49 +0000, Cristian Marussi wrote: > mmm...I dont think you can do this....it is a bit tricky to explain BUT > the optional txdone_irq in the mailbox subsystem is NOT a completion > interrupt as intended in the SCMI world, I'll try to explain why in the > following. > > The SCMI mailbox transport is defined by a mailbox and an associated > shared memory area: when an SCMI client like Linux wants to send a > message to the server it places the message in the shmem, set the > channel BUSY bit, and rings the doorbell to alert the other side; at > this point the SCMI server wakes up grab the message from the shmem, > process it as it sees fit, places the reply back into the same shmem > area, clears the channnel BUSY bit, and CAN optionally ring back the > client with the mailbox doorbell: this completion IRQ is OPTIONAL, and, > if missing, the client will have instead to revert to polling the well > defined channel BUSY bit in the shmem area to know when the reply from > the server is ready. > The end result is that the channel is kept busy until the server has > replied, and cannot be reused until we are sure that the reply is available > AND that the client execution path, waiting for it, has comsumed such > expected reply message. (or times out) > Alternatively, even if we have this completion IRQ we can decide, client > side, to start a polling trabnsaction and dont use the completion > IRQ...but this is another sroty. > > Now, in the mailbox subsystem the txdone_irq, when present is meant to > represnt a txACK IRQ, sent automatically by the mailbox controller, when > the transmission has completed and the issued command has been read by > the server(usually when the server clears some specific reg): it does NOT > assure the client that the server has processed the request and that a > reply has been placed into the shmem area, so it is not what SCMI intend > of a completion interrupt: it is just a mere indication that the > transmission client->server has completed in the mailbox and that the > mailbox channel(doorbell) is now free annd available for another > transmission. > > Indeed, such TxACK, when received by the mailbox driver causes the next > enqueued message to be sent (i.e. the doorbell to be ring), and, while > it is certainly useful in some other context in which you could use such > TxACK to deduce that the channel is free and available for another > transmission, it is not enough in the SCMI world to guarantee that the > SCMI reply has come back and has been processed by the client. > > From another point of view, you could say that the problem is that the > mailbox/doorbell/TxACK is only one side of the story, the other part, > the shared memory area used for the effective message transmission, is > unknown to the mailbox controller and in control of the SCMI stack, so > you cannot let the mailbox subsystem decide when to queue new messages > on the same shared memory area used for cmds and replies. > > In fact, such TxACk interrupt has no practical usage in the SCMI stack > and it is even counterproductive to have it, since it can cause the > client to mistakenly attempt the next transmission before the previous > one has completed, so overwriting the in-flight reply. > (..and our busy-looping client side on the channel BUSY bit does not > help here...) > > The following commit, though, needed mainly for different reasons, should > have indeed solved also the issue with mailbox controllers having a TxACK, > since it introduces a global channel lock that serializes and inhibits any > further transmissions, at the SCMI layer, even on systems that has a working > and enabled TxACK IRQ. > > commit da1642bc97c4ef67f347edcd493bd0a52f88777b (tag: scmi-fixes-6.12) > Author: Justin Chen <justin.chen@broadcom.com> > Date: Mon Oct 14 09:07:17 2024 -0700 > > firmware: arm_scmi: Queue in scmi layer for mailbox implementation > > In general, anyway, it would be even better, if possible, not to enable > at all such txACK IRQ at the mailbox level when such a controller is used > for SCMI: as an example, in arm_mhuv3.c, NOT even defining the txACK in > the DT when describimng the mailbox controller does the trick. > > Did you see any specific anomaly regarding the SCMI stack when using a > mailbox controller which provides a txACK ? > > Sorry for the not so short mail :P ... but if I am missing something > and you are seeing anomalies, please let us know. No need to apologize, and many thanks for taking the time to explain everything in such detail. Much appreciated! What you explained makes perfect sense, and now I clearly understand why the changes I proposed would not work. I have quite the opposite problem. I'm using a mailbox controller that does *not* provide a txACK interrupt (or any kind of interrupt for that matter), and the problem is that SCMI transfers do not complete because scmi_wait_for_reply() times out. I tracked this down to xfer->hdr.poll_completion being false, and therefore scmi_wait_for_reply() taking the second branch, where it calls wait_for_completion_timeout(). Looking at do_xfer(), xfer->hdr.poll_completion is set to true if is_polling_enabled() returns true, and one of the conditions for this to happen is that the no_completion_irq flag in struct scmi_chan_info is set to true. But it defaults to false, and in my case there is nothing there to set it to true. This is what led me to the RFC patch I sent. Even if now that you explained it I understand why the txACK interrupt is different from the SCMI message having been processed and replied, I still fail to understand how it's all supposed to work for the mailbox transport case where the completion IRQ (which you said was optional) is indeed *not* implemented. Since xfer->hdr.poll_completion defaults to false, I looked at what could trigger the completion that scmi_wait_for_reply() waits on, in the case of a mailbox transport. The way it's set up is quite convoluted but if I'm reading the code correctly: * core->rx_callback (in mailbox.c) is set to scmi_rx_callback() * chan->cl->rx_callback is set to rx_callback() (in mailbox.c) Therefore, the (reverse) call path leading to completion would be: complete(&xfer->done) scmi_handle_response() scmi_rx_callback() rx_callback() mbox_chan_received_data() If I understand correctly, mbox_chan_received_data() is supposed to be called by the mailbox provider driver when the mailbox receives data e.g. the SCMI server rings the bell to indicate completion. But doesn't this mean that the completion notification (through mailbox doorbell) is expected to be implemented by default? Now I get that the mailbox layer has no knowledge of the client layer (SCMI in this case) and that the txACK IRQ in the mailbox layer is not meaningful or useful to the SCMI layer. But in my (uneducated) opinion, there should still be a way to set the no_completion_irq flag in struct scmi_chan_info to true for the mailbox transport, for the case when the SCMI server does not implement the completion notification. Perhaps through the device tree? Now it's my turn to apologize for a long email :) Hopefully it makes sense and it's clear now what problem I'm trying to solve. Thanks again for all the input provided so far, and I'm looking forward to some more answers.
diff --git a/drivers/firmware/arm_scmi/transports/mailbox.c b/drivers/firmware/arm_scmi/transports/mailbox.c index b66df29814566..33722f2d82787 100644 --- a/drivers/firmware/arm_scmi/transports/mailbox.c +++ b/drivers/firmware/arm_scmi/transports/mailbox.c @@ -9,6 +9,7 @@ #include <linux/err.h> #include <linux/device.h> #include <linux/mailbox_client.h> +#include <linux/mailbox_controller.h> #include <linux/of.h> #include <linux/of_address.h> #include <linux/platform_device.h> @@ -221,6 +222,9 @@ static int mailbox_chan_setup(struct scmi_chan_info *cinfo, struct device *dev, return ret; } + if (!smbox->chan->mbox->txdone_irq) + cinfo->no_completion_irq = true; + /* Additional unidirectional channel for TX if needed */ if (tx && a2p_rx_chan) { smbox->chan_receiver = mbox_request_channel(cl, a2p_rx_chan);
With the introduction of no_completion_irq in struct scmi_chan_info in commit a690b7e6e774 ("firmware: arm_scmi: Add configurable polling mode for transports") it became possible to enable polling mode by default when the transport does not support completion interrupts. Mailbox controllers on the other hand have a similar mechanism to indicate if completion interrupts are available, using the txdone_irq flag in struct mbox_controller. This is available since the introduction of the mailbox framework in commit 2b6d83e2b8b7 ("mailbox: Introduce framework for mailbox"). Since the mailbox framework is already aware of whether the underlying transport supports completion interrupts or not, set the flag correctly in mailbox_chan_setup() to propagate the information to the SCMI core layer. Without this change, scmi_wait_for_reply() would end up taking the second branch for a mailbox transport with no completion IRQ support, and time out waiting for an interrupt that never comes. Signed-off-by: Radu Rendec <rrendec@redhat.com> --- drivers/firmware/arm_scmi/transports/mailbox.c | 4 ++++ 1 file changed, 4 insertions(+)