| Message ID | 1411808085-27792-7-git-send-email-maxime.ripard@free-electrons.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Sat, Sep 27, 2014 at 10:54:42AM +0200, Maxime Ripard wrote: > dma_slave_caps is very important to the generic layers that might interact with > dmaengine, such as ASoC. Unfortunately, it has been added as yet another > dma_device callback, and most of the existing drivers haven't implemented it, > reducing its reliability. Many haven't implemented it probably because either (a) they don't get used with ASoC, or (b) they aren't aware of the new interface, or (c) can't be bothered with the churn. However, trying to return something introduces a bug: > static inline int dma_get_slave_caps(struct dma_chan *chan, struct dma_slave_caps *caps) > { > + struct dma_device *device; > + > if (!chan || !caps) > return -EINVAL; > > + device = chan->device; > + > /* check if the channel supports slave transactions */ > - if (!test_bit(DMA_SLAVE, chan->device->cap_mask.bits)) > + if (!test_bit(DMA_SLAVE, device->cap_mask.bits)) > return -ENXIO; > > - if (chan->device->device_slave_caps) > - return chan->device->device_slave_caps(chan, caps); > + caps->cmd_pause = !!device->device_pause; > + caps->cmd_terminate = !!device->device_terminate_all; > + > + if (device->device_slave_caps) > + return device->device_slave_caps(chan, caps); > > - return -ENXIO; > + return 0; So this now returns success if the driver doesn't implement device_slave_caps(), but with most of the structure zero. Now, consider what effect this has with: ret = dma_get_slave_caps(chan, &dma_caps); if (ret == 0) { if (dma_caps.cmd_pause) hw.info |= SNDRV_PCM_INFO_PAUSE | SNDRV_PCM_INFO_RESUME; if (dma_caps.residue_granularity <= DMA_RESIDUE_GRANULARITY_SEGMENT) hw.info |= SNDRV_PCM_INFO_BATCH; if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) addr_widths = dma_caps.dstn_addr_widths; else addr_widths = dma_caps.src_addr_widths; } addr_widths becomes zero, and we also get SNDRV_PCM_INFO_BATCH turned on for _all_ DMA engine drivers. The first renders ASoC useless with DMA engine. It may be a good way to get people to implement it, but this will cause regressions.
Hi Russell, On Sat, Sep 27, 2014 at 10:25:36AM +0100, Russell King - ARM Linux wrote: > On Sat, Sep 27, 2014 at 10:54:42AM +0200, Maxime Ripard wrote: > > dma_slave_caps is very important to the generic layers that might interact with > > dmaengine, such as ASoC. Unfortunately, it has been added as yet another > > dma_device callback, and most of the existing drivers haven't implemented it, > > reducing its reliability. > > Many haven't implemented it probably because either (a) they don't get used > with ASoC, or (b) they aren't aware of the new interface, or (c) can't be > bothered with the churn. For a), I really see this as a chicken-egg issue. ASoC is the only user of it because it's the only framework that has a generic layer on top, and it's the only framework that has a generic layer because most drivers don't implement it. Now, there seems to be a trend to actually use a generic DMA layer in other frameworks. SPI gained one recently, I think I saw something about some discussions for IIO and I2C too. And in order for this to work, we have to make it reliable, and as such, implemented on most drivers. > However, trying to return something introduces a bug: > > > static inline int dma_get_slave_caps(struct dma_chan *chan, struct dma_slave_caps *caps) > > { > > + struct dma_device *device; > > + > > if (!chan || !caps) > > return -EINVAL; > > > > + device = chan->device; > > + > > /* check if the channel supports slave transactions */ > > - if (!test_bit(DMA_SLAVE, chan->device->cap_mask.bits)) > > + if (!test_bit(DMA_SLAVE, device->cap_mask.bits)) > > return -ENXIO; > > > > - if (chan->device->device_slave_caps) > > - return chan->device->device_slave_caps(chan, caps); > > + caps->cmd_pause = !!device->device_pause; > > + caps->cmd_terminate = !!device->device_terminate_all; > > + > > + if (device->device_slave_caps) > > + return device->device_slave_caps(chan, caps); > > > > - return -ENXIO; > > + return 0; > > So this now returns success if the driver doesn't implement device_slave_caps(), > but with most of the structure zero. > > Now, consider what effect this has with: > > > ret = dma_get_slave_caps(chan, &dma_caps); > if (ret == 0) { > if (dma_caps.cmd_pause) > hw.info |= SNDRV_PCM_INFO_PAUSE | SNDRV_PCM_INFO_RESUME; > if (dma_caps.residue_granularity <= DMA_RESIDUE_GRANULARITY_SEGMENT) > hw.info |= SNDRV_PCM_INFO_BATCH; > > if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) > addr_widths = dma_caps.dstn_addr_widths; > else > addr_widths = dma_caps.src_addr_widths; > } > > addr_widths becomes zero, and we also get SNDRV_PCM_INFO_BATCH turned > on for _all_ DMA engine drivers. The first renders ASoC useless with > DMA engine. > > It may be a good way to get people to implement it, but this will cause > regressions. Hmmm, nasty indeed. Maybe we could add a test to see if any of the field we're going to use are filled, and if not, return an error? Thanks! Maxime
diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h index 4d0294ec3567..cfcbee145898 100644 --- a/include/linux/dmaengine.h +++ b/include/linux/dmaengine.h @@ -772,17 +772,24 @@ static inline struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma( static inline int dma_get_slave_caps(struct dma_chan *chan, struct dma_slave_caps *caps) { + struct dma_device *device; + if (!chan || !caps) return -EINVAL; + device = chan->device; + /* check if the channel supports slave transactions */ - if (!test_bit(DMA_SLAVE, chan->device->cap_mask.bits)) + if (!test_bit(DMA_SLAVE, device->cap_mask.bits)) return -ENXIO; - if (chan->device->device_slave_caps) - return chan->device->device_slave_caps(chan, caps); + caps->cmd_pause = !!device->device_pause; + caps->cmd_terminate = !!device->device_terminate_all; + + if (device->device_slave_caps) + return device->device_slave_caps(chan, caps); - return -ENXIO; + return 0; } static inline int dmaengine_terminate_all(struct dma_chan *chan)
dma_slave_caps is very important to the generic layers that might interact with dmaengine, such as ASoC. Unfortunately, it has been added as yet another dma_device callback, and most of the existing drivers haven't implemented it, reducing its reliability. Introduce a generic behaviour, pre-filling the dma_slave_caps struct with what it knows about the dma controller, before calling the dma_device callback, both to maintain a backward compatibility and to make sure that drivers can override that generic behaviour. Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com> --- include/linux/dmaengine.h | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-)