linux-kernel - Re: [PATCH v2 2/2] dmaengine: uniphier-mdmac: add UniPhier MIO DMAC driver

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Date:   Wed, 12 Sep 2018 10:05:30 +0530
From:   Vinod <vkoul@...nel.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     "open list:DMA GENERIC OFFLOAD ENGINE SUBSYSTEM" 
        <dmaengine@...r.kernel.org>, DTML <devicetree@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 2/2] dmaengine: uniphier-mdmac: add UniPhier MIO DMAC
 driver

On 12-09-18, 12:01, Masahiro Yamada wrote:
> Hi Vinod,
> 
> 
> 2018-09-11 16:00 GMT+09:00 Vinod <vkoul@...nel.org>:
> > On 24-08-18, 10:41, Masahiro Yamada wrote:
> >
> >> +/* mc->vc.lock must be held by caller */
> >> +static u32 __uniphier_mdmac_get_residue(struct uniphier_mdmac_desc *md)
> >> +{
> >> +     u32 residue = 0;
> >> +     int i;
> >> +
> >> +     for (i = md->sg_cur; i < md->sg_len; i++)
> >> +             residue += sg_dma_len(&md->sgl[i]);
> >
> > so if the descriptor is submitted to hardware, we return the descriptor
> > length, which is not correct.
> >
> > Two cases are required to be handled:
> > 1. Descriptor is in queue (IMO above logic is fine for that, but it can
> > be calculated at descriptor submit and looked up here)
> 
> Where do you want it to be calculated?

where is it calculated now?

> This hardware provides only simple registers (address and size)
> for one-shot transfer instead of descriptors.
> 
> So, I used sgl as-is because I did not see a good reason
> to transform sgl to another data structure.


> > this seems missing stuff. Where do you do register calculation for the
> > descriptor and where is slave_config here, how do you know where to
> > send/receive data form/to (peripheral)
> 
> 
> This dmac is really simple, and un-flexible.
> 
> The peripheral address to send/receive data from/to is hard-weird.
> cfg->{src_addr,dst_addr} is not configurable.
> 
> Look at __uniphier_mdmac_handle().
> 'dest_addr' and 'src_addr' must be set to 0 for the peripheral.

Fair enough, what about other values like addr_width and maxburst?
-- 
~Vinod