| Message ID | 20200526225022.20405-11-Sergey.Semin@baikalelectronics.ru (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | dmaengine: dw: Take Baikal-T1 SoC DW DMAC peculiarities into account | expand |
On Wed, May 27, 2020 at 01:50:21AM +0300, Serge Semin wrote: > Multi-block support provides a way to map the kernel-specific SG-table so > the DW DMA device would handle it as a whole instead of handling the > SG-list items or so called LLP block items one by one. So if true LLP > list isn't supported by the DW DMA engine, then soft-LLP mode will be > utilized to load and execute each LLP-block one by one. The soft-LLP mode > of the DMA transactions execution might not work well for some DMA > consumers like SPI due to its Tx and Rx buffers inter-dependency. Let's > expose the nollp flag indicating the soft-LLP mode by means of the > max_sg_nents capability, so the DMA consumer would be ready to somehow > workaround errors caused by such mode being utilized. > In principal I agree, one nit below. If you are okay with it, feel free to add my Rb tag. > Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru> > Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru> > Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> > Cc: Arnd Bergmann <arnd@arndb.de> > Cc: Rob Herring <robh+dt@kernel.org> > Cc: linux-mips@vger.kernel.org > Cc: devicetree@vger.kernel.org > > --- > > Changelog v3: > - This is a new patch created as a result of the discussion with Vinud and > Andy in the framework of DW DMA burst and LLP capabilities. > --- > drivers/dma/dw/core.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c > index 29c4ef08311d..b850eb7fd084 100644 > --- a/drivers/dma/dw/core.c > +++ b/drivers/dma/dw/core.c > @@ -1054,6 +1054,15 @@ static void dwc_caps(struct dma_chan *chan, struct dma_slave_caps *caps) > struct dw_dma_chan *dwc = to_dw_dma_chan(chan); > > caps->max_burst = dwc->max_burst; > + > + /* > + * It might be crucial for some devices to have the hardware > + * accelerated multi-block transfers supported, aka LLPs in DW DMAC > + * notation. So if LLPs are supported then max_sg_nents is set to > + * zero which means unlimited number of SG entries can be handled in a > + * single DMA transaction, otherwise it's just one SG entry. > + */ > + caps->max_sg_nents = dwc->nollp; To be on the safer side I would explicitly do it like if (dwc->nollp) /* your nice comment */ = 1; else /* Unlimited */ = 0; type or content of nollp theoretically can be changed and this will affect maximum segments. > } > > int do_dma_probe(struct dw_dma_chip *chip) > -- > 2.26.2 >
On Thu, May 28, 2020 at 05:56:30PM +0300, Andy Shevchenko wrote: > On Wed, May 27, 2020 at 01:50:21AM +0300, Serge Semin wrote: > > Multi-block support provides a way to map the kernel-specific SG-table so > > the DW DMA device would handle it as a whole instead of handling the > > SG-list items or so called LLP block items one by one. So if true LLP > > list isn't supported by the DW DMA engine, then soft-LLP mode will be > > utilized to load and execute each LLP-block one by one. The soft-LLP mode > > of the DMA transactions execution might not work well for some DMA > > consumers like SPI due to its Tx and Rx buffers inter-dependency. Let's > > expose the nollp flag indicating the soft-LLP mode by means of the > > max_sg_nents capability, so the DMA consumer would be ready to somehow > > workaround errors caused by such mode being utilized. > > > > In principal I agree, one nit below. > If you are okay with it, feel free to add my Rb tag. > > > Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru> > > Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru> > > Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> > > Cc: Arnd Bergmann <arnd@arndb.de> > > Cc: Rob Herring <robh+dt@kernel.org> > > Cc: linux-mips@vger.kernel.org > > Cc: devicetree@vger.kernel.org > > > > --- > > > > Changelog v3: > > - This is a new patch created as a result of the discussion with Vinud and > > Andy in the framework of DW DMA burst and LLP capabilities. > > --- > > drivers/dma/dw/core.c | 9 +++++++++ > > 1 file changed, 9 insertions(+) > > > > diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c > > index 29c4ef08311d..b850eb7fd084 100644 > > --- a/drivers/dma/dw/core.c > > +++ b/drivers/dma/dw/core.c > > @@ -1054,6 +1054,15 @@ static void dwc_caps(struct dma_chan *chan, struct dma_slave_caps *caps) > > struct dw_dma_chan *dwc = to_dw_dma_chan(chan); > > > > caps->max_burst = dwc->max_burst; > > + > > + /* > > + * It might be crucial for some devices to have the hardware > > + * accelerated multi-block transfers supported, aka LLPs in DW DMAC > > + * notation. So if LLPs are supported then max_sg_nents is set to > > + * zero which means unlimited number of SG entries can be handled in a > > + * single DMA transaction, otherwise it's just one SG entry. > > + */ > > > + caps->max_sg_nents = dwc->nollp; > > To be on the safer side I would explicitly do it like > > if (dwc->nollp) > /* your nice comment */ > = 1; > else > /* Unlimited */ > = 0; > > type or content of nollp theoretically can be changed and this will affect maximum segments. Agree. Though I don't like formatting you suggested. If I add my nice comment between if-statement and assignment the the former will be look detached from the if-statement, which seems a bit ugly. So I'd leave the comment above the whole if-else statement, especially seeing I've already mentioned there about the unlimited number of SG entries there. /* * It might be crucial for some devices to have the hardware * accelerated multi-block transfers supported, aka LLPs in DW DMAC * notation. So if LLPs are supported then max_sg_nents is set to * zero which means unlimited number of SG entries can be handled in a * single DMA transaction, otherwise it's just one SG entry. */ if (dwc->nollp) caps->max_sg_nents = 1; else caps->max_sg_nents = 0; -Sergey > > > } > > > > int do_dma_probe(struct dw_dma_chip *chip) > > -- > > 2.26.2 > > > > -- > With Best Regards, > Andy Shevchenko > >
On Thu, May 28, 2020 at 6:52 PM Serge Semin <Sergey.Semin@baikalelectronics.ru> wrote: > On Thu, May 28, 2020 at 05:56:30PM +0300, Andy Shevchenko wrote: > > On Wed, May 27, 2020 at 01:50:21AM +0300, Serge Semin wrote: ... > > In principal I agree, one nit below. > > If you are okay with it, feel free to add my Rb tag. > > > + /* > > > + * It might be crucial for some devices to have the hardware > > > + * accelerated multi-block transfers supported, aka LLPs in DW DMAC > > > + * notation. So if LLPs are supported then max_sg_nents is set to > > > + * zero which means unlimited number of SG entries can be handled in a > > > + * single DMA transaction, otherwise it's just one SG entry. > > > + */ > > > > > + caps->max_sg_nents = dwc->nollp; > > > > > To be on the safer side I would explicitly do it like > > > > if (dwc->nollp) > > /* your nice comment */ > > = 1; > > else > > /* Unlimited */ > > = 0; > > > > type or content of nollp theoretically can be changed and this will affect maximum segments. > > Agree. Though I don't like formatting you suggested. If I add my nice comment > between if-statement and assignment the the former will be look detached from > the if-statement, which seems a bit ugly. So I'd leave the comment above the > whole if-else statement, especially seeing I've already mentioned there about > the unlimited number of SG entries there. > > /* > * It might be crucial for some devices to have the hardware > * accelerated multi-block transfers supported, aka LLPs in DW DMAC > * notation. So if LLPs are supported then max_sg_nents is set to > * zero which means unlimited number of SG entries can be handled in a > * single DMA transaction, otherwise it's just one SG entry. > */ > if (dwc->nollp) > caps->max_sg_nents = 1; > else > caps->max_sg_nents = 0; Fine with me, thanks!
diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c index 29c4ef08311d..b850eb7fd084 100644 --- a/drivers/dma/dw/core.c +++ b/drivers/dma/dw/core.c @@ -1054,6 +1054,15 @@ static void dwc_caps(struct dma_chan *chan, struct dma_slave_caps *caps) struct dw_dma_chan *dwc = to_dw_dma_chan(chan); caps->max_burst = dwc->max_burst; + + /* + * It might be crucial for some devices to have the hardware + * accelerated multi-block transfers supported, aka LLPs in DW DMAC + * notation. So if LLPs are supported then max_sg_nents is set to + * zero which means unlimited number of SG entries can be handled in a + * single DMA transaction, otherwise it's just one SG entry. + */ + caps->max_sg_nents = dwc->nollp; } int do_dma_probe(struct dw_dma_chip *chip)
Multi-block support provides a way to map the kernel-specific SG-table so the DW DMA device would handle it as a whole instead of handling the SG-list items or so called LLP block items one by one. So if true LLP list isn't supported by the DW DMA engine, then soft-LLP mode will be utilized to load and execute each LLP-block one by one. The soft-LLP mode of the DMA transactions execution might not work well for some DMA consumers like SPI due to its Tx and Rx buffers inter-dependency. Let's expose the nollp flag indicating the soft-LLP mode by means of the max_sg_nents capability, so the DMA consumer would be ready to somehow workaround errors caused by such mode being utilized. Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru> Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru> Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Rob Herring <robh+dt@kernel.org> Cc: linux-mips@vger.kernel.org Cc: devicetree@vger.kernel.org --- Changelog v3: - This is a new patch created as a result of the discussion with Vinud and Andy in the framework of DW DMA burst and LLP capabilities. --- drivers/dma/dw/core.c | 9 +++++++++ 1 file changed, 9 insertions(+)