linux-kernel - Re: [PATCH v7 05/11] dmaengine: Introduce DMA-device device

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Date:   Tue, 14 Jul 2020 19:29:53 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Dave Jiang <dave.jiang@...el.com>
Cc:     Vinod Koul <vkoul@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Viresh Kumar <vireshk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, linux-mips@...r.kernel.org,
        devicetree@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 05/11] dmaengine: Introduce DMA-device device_caps
 callback

On Tue, Jul 14, 2020 at 09:18:16AM -0700, Dave Jiang wrote:
> 
> 
> On 7/14/2020 9:08 AM, Vinod Koul wrote:
> > On 13-07-20, 13:55, Dave Jiang wrote:
> > > 
> > > 
> > > On 7/10/2020 2:38 AM, Serge Semin wrote:
> > > > On Fri, Jul 10, 2020 at 11:45:03AM +0300, Andy Shevchenko wrote:
> > > > > On Fri, Jul 10, 2020 at 01:45:44AM +0300, Serge Semin wrote:
> > > > > > There are DMA devices (like ours version of Synopsys DW DMAC) which have
> > > > > > DMA capabilities non-uniformly redistributed between the device channels.
> > > > > > In order to provide a way of exposing the channel-specific parameters to
> > > > > > the DMA engine consumers, we introduce a new DMA-device callback. In case
> > > > > > if provided it gets called from the dma_get_slave_caps() method and is
> > > > > > able to override the generic DMA-device capabilities.
> > > > > 
> > > > 
> > > > > In light of recent developments consider not to add 'slave' and a such words to the kernel.
> > > > 
> > > > As long as the 'slave' word is used in the name of the dma_slave_caps
> > > > structure and in the rest of the DMA-engine subsystem, it will be ambiguous
> > > > to use some else terminology. If renaming needs to be done, then it should be
> > > > done synchronously for the whole subsystem.
> > > 
> > > What about just calling it dma_device_caps? Consider this is a useful
> > > function not only slave DMA will utilize this. I can see this being useful
> > > for some of my future code with idxd driver.
> > 
> > Some of the caps may make sense to generic dmaengine but few of them do
> > not :) While at it, am planning to make it dmaengine_periph_caps to
> > denote that these are dmaengine peripheral capabilities.
> > 
> 

> If the function only passes in periph_caps, how do we allow the non periph
> DMA utilize this function?

Hello Dave. That seems reasonable. "dma_device_caps" or even "dma_chan_caps"
might be more suitable seeing after this patchset merged in the "dma_slave_caps"
may really provide the DMA channel-specific configs. Moreover that structure is
accessible only by means of the dma_chan descriptor:

int dma_get_slave_caps(struct dma_chan *chan, struct dma_slave_caps *caps);

which makes those caps being the channel-specific even without this patchset.

So as I see it "dma_chan_caps" might be the better choice.

-Sergey