Re: [PATCH] dma: edma: add device_slave_caps() support

[Lars-Peter Clausen]

On 07/24/2013 10:28 AM, Fernandes, Joel wrote:
> 
> On Jul 24, 2013, at 3:23 AM, "Lars-Peter Clausen" <lars@xxxxxxxxxx> wrote:
> 
>> On 07/24/2013 10:11 AM, Joel Fernandes wrote:
>>> On 07/24/2013 03:03 AM, Lars-Peter Clausen wrote:
>>>> On 07/23/2013 06:43 PM, Joel Fernandes wrote:
>>>>> Implement device_slave_caps(). EDMA has a limited number of slots.
>>>>> Slave drivers such as omap_hsmmc will query the driver to make
>>>>> sure they don't pass in more than these many scatter segments.
>>>>>
>>>>> Signed-off-by: Joel Fernandes <joelf@xxxxxx>
>>>>> ---
>>>>> Vinod, or Dan- If this patch looks ok, can you please merge in for
>>>>> -rc cycle? This patch is required to fix MMC support on AM33xx. This
>>>>> patch is blocking 3 other patches which fix various MMC things. Thanks!
>>>>>
>>>>> Notes:
>>>>> (1) this approach is temporary and only for -rc cycle to fix MMC on
>>>>> AM335x. It will be replace by the RFC series in future kernels:
>>>>> http://www.spinics.net/lists/arm-kernel/msg260094.html
>>>>>
>>>>> (2) Patch depends Vinod's patch at:
>>>>> http://permalink.gmane.org/gmane.linux.kernel/1525112
>>>>>
>>>>> drivers/dma/edma.c |    9 +++++++++
>>>>> 1 file changed, 9 insertions(+)
>>>>>
>>>>> diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c
>>>>> index 7222cbe..81d5429 100644
>>>>> --- a/drivers/dma/edma.c
>>>>> +++ b/drivers/dma/edma.c
>>>>> @@ -517,6 +517,14 @@ static void edma_issue_pending(struct dma_chan *chan)
>>>>>    spin_unlock_irqrestore(&echan->vchan.lock, flags);
>>>>> }
>>>>>
>>>>> +static inline int edma_slave_caps(struct dma_chan *chan,
>>>>> +    struct dma_slave_caps *caps)
>>>>> +{
>>>>> +    caps->max_sg_nr = MAX_NR_SG;
>>>>
>>>> Hm, what about the other fields?
>>>
>>> Other fields are unused, the max segment size is supposed to be
>>> calculated "given" the address width and burst size. Since these
>>> can't be provided to get_caps, I have left it out for now.
>>> See: https://lkml.org/lkml/2013/3/6/464
>>
>> The PL330 driver is similar in this regard, the maximum segment size also
>> depends on address width and burst width. What I did for the get_slave_caps
>> implementation is to set it to the minimum maximum size. E.g. in you case
>> that should be SZ_64K - 1 (burstsize and addrwidth both set to 1).
> 
> So you're setting max to minimum maximum size? Isn't that like telling the driver that its segments can't be bigger than this... Unless I'm missing something..

Yes. This is a limitation of the current slave_caps API. The maximum needs
to be the maximum for all possible configurations. A specific configuration
may allow a larger maximum. So we maybe have to extend the API to be able to
query the limits for a certain configuration. Not sure what the best way
would be to do that, either adding a config parameter to get_slave_caps or
to break it into two functions like you proposed one for the static
capabilities and one for the sg limits.

> 
>>
>>>
>>> Even if it did, the "segment size" itself is unused in the MMC driver
>>> that this is supposed to fix, unlike the "number of segments" which I'm
>>> populating above.
>>
>> E.g. for ALSA we'll need to know the max segment size, so I think it doesn't
>> hurt add this in this patch as well.
> 
> For alsa it would dma only the minimum max size even if the dma controller could do more?
> 

Yes, but I think 64k is still plenty enough for the max period size. The
current davinci PCM driver even claims to only support up to 8k.

- Lars
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