Re: [PATCH] mmc: core: Lower max_seg_size if too high for DMA

[Peter Ujfalusi <peter.ujfalusi@ti.com>]

Russell,

On 11/12/2018 15.49, Russell King - ARM Linux wrote:
>>> The only thing I can think of doing is adding into omap_hsmmc:
>>>
>>>         mmc->max_seg_size = min(mmc->max_req_size,
>>>                                 min(dma_get_max_seg_size(host->rx_chan->device->dev),
>>>                                     dma_get_max_seg_size(host->tx_chan->device->dev)));
>>>
>>> to limit the maximum segment size to that of the device _and_ dma
>>> engine's capabilities.
>>>
>>> Doing this solves the problem for me.
>>
>> Thanks for the suggestion - it sounds like a reasonable way to fix the
>> problem, at least for now.
> 
> I don't think there's any "at least for now" about this approach - it's
> not something that the MMC core can know about, because whether a driver
> uses DMA engine or not, and how many channels it uses is completely
> driver specific.

I agree.

> The only questionable thing is around the dma_get_max_seg_size()
> interface, but the only way that's going to get solved is to eliminate
> it entirely as it isn't a per-device property with some DMA engines
> such as omap-dma - it's a per-request property.  That also means
> killing the check in the DMA debug code, which isn't going to go down
> very well.
> 
>> Do you want to send to patch or do you expect someone else to do it?
> 
> I'll send a patch once I've checked the corner cases, and whether
> we should go further and include other DMA parameters from the
> dma engine.

There is one thing which I'm trying to figure out:
sDMA (and EDMA also) have limitation on the length of each sg segment.

To make things a bit more interesting the limit is not in bytes, but in
number of bursts:

65535 * burst * addr_width = max_sg_segment_len in bytes.

We can not set this to a static value as depending on the addr_width and
burst it can be different for each transfer.

- Péter

Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki

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