Re: [PATCH] dmaengine: dma_request_chan_by_mask() defer probing unconditionally

[Vignesh Raghavendra <vigneshr@ti.com>]

Hi Enric,

On 29/11/24 22:14, Robin Murphy wrote:
> On 2024-11-27 8:23 am, Enric Balletbo i Serra wrote:
>> Having no DMA devices registered is not a guarantee that the device
>> doesn't exist, it could be that is not registered yet, so return
>> EPROBE_DEFER unconditionally so the caller can wait for the required
>> DMA device registered.
>>
>> Signed-off-by: Enric Balletbo i Serra <eballetb@redhat.com>
>> ---
>> This patch fixes the following error on TI AM69-SK
>>
>> [    2.854501] cadence-qspi 47040000.spi: error -ENODEV: No Rx DMA
>> available
>>
>> The DMA device is probed after cadence-qspi driver, so deferring it
>> solves the problem.
> 
> Conversely, though, it does carry some risk that if there really is no
> DMA device/driver, other callers (e.g. spi-ti-qspi) may now get stuck
> deferring forever where the -ENODEV would have let them proceed with a
> fallback to non-DMA operation. driver_deferred_probe_check_state() is
> typically a good tool for these situations, but I guess it's a bit
> tricky in a context where we don't actually have the dependent device to
> hand :/


+1. There is no explicit dependency that can be modeled (via DT or
otherwise) for memcpy DMA channels. And the IP (cadence-qspi) is not
specific to TI platforms. Its very much possible that a non TI platform
may not have memcpy DMA channel at all. Wont this end up breaking such
platforms wrt using SPI flash using CPU bound IO?

> 
> Thanks,
> Robin.
> 
>> ---
>>   drivers/dma/dmaengine.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
>> index
>> c1357d7f3dc6ca7899c4d68a039567e73b0f089d..57f07b477a5d9ad8f2656584b8c0d6dffb2ab469 100644
>> --- a/drivers/dma/dmaengine.c
>> +++ b/drivers/dma/dmaengine.c
>> @@ -889,10 +889,10 @@ struct dma_chan *dma_request_chan_by_mask(const
>> dma_cap_mask_t *mask)
>>       chan = __dma_request_channel(mask, NULL, NULL, NULL);
>>       if (!chan) {
>>           mutex_lock(&dma_list_mutex);
>> -        if (list_empty(&dma_device_list))
>> -            chan = ERR_PTR(-EPROBE_DEFER);
>> -        else
>> -            chan = ERR_PTR(-ENODEV);
>> +        /* If the required DMA device is not registered yet,
>> +         * return EPROBE_DEFER
>> +         */
>> +        chan = ERR_PTR(-EPROBE_DEFER);
>>           mutex_unlock(&dma_list_mutex);
>>       }
>>  
>> ---
>> base-commit: 43fb83c17ba2d63dfb798f0be7453ed55ca3f9c2
>> change-id: 20241127-defer-dma-request-chan-4f26c62c8691
>>
>> Best regards,
> 

-- 
Regards
Vignesh
https://ti.com/opensource