Re: [PATCH v2 4/6] dmaengine: dw: Print warning if multi-block is unsupported

[Serge Semin <Sergey.Semin@baikalelectronics.ru>]

On Tue, May 19, 2020 at 10:32:46PM +0530, Vinod Koul wrote:
> On 17-05-20, 22:23, Serge Semin wrote:
> > On Fri, May 15, 2020 at 12:00:39PM +0530, Vinod Koul wrote:
> > > Hi Serge,
> > > 
> > > On 12-05-20, 15:42, Serge Semin wrote:
> > > > Vinod,
> > > > 
> > > > Could you join the discussion for a little bit?
> > > > 
> > > > In order to properly fix the problem discussed in this topic, we need to
> > > > introduce an additional capability exported by DMA channel handlers on per-channel
> > > > basis. It must be a number, which would indicate an upper limitation of the SG list
> > > > entries amount.
> > > > Something like this would do it:
> > > > struct dma_slave_caps {
> > > > ...
> > > > 	unsigned int max_sg_nents;
> > > > ...
> > > 
> > > Looking at the discussion, I agree we should can this up in the
> > > interface. The max_dma_len suggests the length of a descriptor allowed,
> > > it does not convey the sg_nents supported which in the case of nollp is
> > > one.
> > > 
> > > Btw is this is a real hardware issue, I have found that value of such
> > > hardware is very less and people did fix it up in subsequent revs to add
> > > llp support.
> > 
> > Yes, it is. My DW DMAC doesn't support LLP and there isn't going to be new SoC
> > version produced.(
> 
> Ouch
> 
> > > Also, another question is why this cannot be handled in driver, I agree
> > > your hardware does not support llp but that does not stop you from
> > > breaking a multi_sg list into N hardware descriptors and keep submitting
> > > them (for this to work submission should be done in isr and not in bh,
> > > unfortunately very few driver take that route).
> > 
> > Current DW DMA driver does that, but this isn't enough. The problem is that
> > in order to fix the issue in the DMA hardware driver we need to introduce
> > an inter-dependent channels abstraction and synchronously feed both Tx and
> > Rx DMA channels with hardware descriptors (LLP entries) one-by-one. This hardly
> > needed by any slave device driver rather than SPI, which Tx and Rx buffers are
> > inter-dependent. So Andy's idea was to move the fix to the SPI driver (feed
> > the DMA engine channels with Tx and Rx data buffers synchronously), but DMA
> > engine would provide an info whether such fix is required. This can be
> > determined by the maximum SG entries capability.
> 
> Okay but having the sw limitation removed would also be a good idea, you
> can handle any user, I will leave it upto you, either way is okay
> 
> > 
> > (Note max_sg_ents isn't a limitation on the number of SG entries supported by
> > the DMA driver, but the number of SG entries handled by the DMA engine in a
> > single DMA transaction.)
> > 
> > > TBH the max_sg_nents or
> > > max_dma_len are HW restrictions and SW *can* deal with then :-)
> > 
> > Yes, it can, but it only works for the cases when individual DMA channels are
> > utilized. DMA hardware driver doesn't know that the target and source slave
> > device buffers (SPI Tx and Rx FIFOs) are inter-dependent, that writing to one
> > you will implicitly push data to another. So due to the interrupts handling
> > latency Tx DMA channel is restarted faster than Rx DMA channel is reinitialized.
> > This causes the SPI Rx FIFO overflow and data loss.
> > 
> > > 
> > > In an idea world, you should break the sw descriptor submitted into N hw
> > > descriptors and submit to hardware and let user know when the sw
> > > descriptor is completed. Of course we do not do that :(
> > 
> > Well, the current Dw DMA driver does that. But due to the two slave device
> > buffers inter-dependency this isn't enough to perform safe DMA transactions.
> > Due to the interrupts handling latency Tx DMA channel pushes data to the slave
> > device buffer faster than Rx DMA channel starts to handle incoming data. This
> > causes the SPI Rx FIFO overflow.
> > 
> > > 
> > > > };
> > > > As Andy suggested it's value should be interpreted as:
> > > > 0          - unlimited number of entries,
> > > > 1:MAX_UINT - actual limit to the number of entries.
> > > 
> > 
> > > Hmm why 0, why not MAX_UINT for unlimited?
> > 
> > 0 is much better for many reasons. First of all MAX_UINT is a lot, but it's
> > still a number. On x64 platform this might be actual limit if for instance
> > the block-size register is 32-bits wide. Secondly interpreting 0 as unlimited
> > number of entries would be more suitable since most of the drivers support
> > LLP functionality and we wouldn't need to update their code to set MAX_UINT.
> > Thirdly DMA engines, which don't support LLPs would need to set this parameter
> > as 1. So if we do as you say and interpret unlimited number of LLPs as MAX_UINT,
> > then 0 would left unused.
> > 
> > To sum up I also think that using 0 as unlimited number SG entries supported is
> > much better.
> 
> ok
> 
> > > > In addition to that seeing the dma_get_slave_caps() method provide the caps only
> > > > by getting them from the DMA device descriptor, while we need to have an info on
> > > > per-channel basis, it would be good to introduce a new DMA-device callback like:
> > > > struct dma_device {
> > > > ...
> > > > 	int (*device_caps)(struct dma_chan *chan,
> > > > 			   struct dma_slave_caps *caps);
> > > 
> > 
> > > Do you have a controller where channel caps are on per-channel basis?
> > 
> > Yes, I do. Our DW DMA controller has got the maximum burst length non-uniformly
> > distributed per DMA channels. There are eight channels our controller supports,
> > among which first two channels can burst up to 32 transfer words, but the rest
> > of the channels support bursting up to 4 transfer words.
> > 
> > So having such device_caps() callback to customize the device capabilities on
> > per-DMA-channel basis would be very useful! What do you think?
> 
> Okay looks like per-ch basis is the way forward!

Great! Thanks. I'll send v3 with updates we've come up to in this discussion.

-Sergey

> 
> -- 
> ~Vinod