[PATCH 3/5] dmaengine: Support for querying maximum trasnfer length (of an SG element)

[Vinod Koul]

On Mon, Oct 02, 2017 at 02:24:12PM +0300, Peter Ujfalusi wrote:
> 
> 
> 
> Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki. Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
> 
> On 2017-09-26 19:54, Vinod Koul wrote:
> >>>
> >>> not another callback :)
> >>>
> >>> on a serious note, why shouldn't this be one more capability in
> >>> dma_slave_caps. looking at next patch it seems static
> >>
> >> It is not really static, the size in bytes depends on the dev_width and
> >> the maxburst:
> >> dev_width * burst * (SZ_64K - 1);
> > 
> > well DMAengines work on FIFOs, in above you are giving length as SZ_64K - 1
> > 'items' which IIUC in DMAengine terms for bytes would always refer wrt width
> > used and burst applied.
> 
> I think we can live with this and let the user to figure out what to do
> with this information.

Right, plus a macro for conversion :) SO that users dont code buggy
conversions all over the place

> But I'm having hard time to figure out a good name for this. It is not
> the number of SGs we can support, but the number of 'items' within one
> SG that we have the limit. It could be:
> u32 max_bursts_per_sg;

this looks fine, another candidate I would use is words_per_sg and while at
it why tie it to sg? should we make it words_per_txn but then people should not
confuse with txn represented by a descriptor which can have multiple ....

> 
> which would also apply to period length (for cyclic) in a similar way.
> 
> > Return length in bytes does make sense (from user PoV), but then you need to
> > "know" the applied  width and burst. How do you decide those?
> 
> The number of items works eDMA and sDMA, but we also have the cpp41. It
> is a packet DMA and it has no understanding of bursts, address widths or
> any of the 'traditional' things. It only cares about the number of bytes
> we want to transfer and it has limitation of 4194303 bytes (21bits for
> length). This is again per SG. How this could report the
> 'max_bursts_per_sg' ?

hmmm that is intresting case, is this number coming from USB side?

> This was one of the reasons that I have settled with the callback.
> 
> What we can also do is to code this within the DMA drivers itself.
> 
> When setting up the transfer and we realize that one of the SG will not
> going to fit, we destroy what we have done so far, pass the sg list
> along with length/sg limit to create a new sg list where all sg item's
> length is under the limit. Then using this new sg list we can set up the
> transfer.
> 
> I'm not sure how hard is to do the sg list optimization, I see that
> sg_split() is not what we want so we might need to code this in
> dmaengine or in the scatterlist code.
> 
> We certainly don't want to verify all slave_sg transfers proactively to
> avoid adding latency when it is not necessary.

latency would be added at prepare, not when submitting..

-- 
~Vinod