linux-kernel - Re: [PATCH v7 04/11] dmaengine: Introduce max SG list entries capability

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Message-ID: <20200717081403.GL82923@vkoul-mobl>
Date:   Fri, 17 Jul 2020 13:44:03 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Serge Semin <fancer.lancer@...il.com>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Viresh Kumar <vireshk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.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 04/11] dmaengine: Introduce max SG list entries
 capability

On 15-07-20, 20:08, Serge Semin wrote:
> On Wed, Jul 15, 2020 at 04:43:15PM +0530, Vinod Koul wrote:
> > On 10-07-20, 19:14, Serge Semin wrote:
> > > On Fri, Jul 10, 2020 at 02:51:33PM +0300, Peter Ujfalusi wrote:
> > 
> > > > Since we should be able to handle longer lists and this is kind of a
> > > > hint for clients that above this number of nents the list will be broken
> > > > up to smaller 'bursts', which when traversing could cause latency.
> > > > 
> > > > sg_chunk_len might be another candidate.
> > > 
> > > Ok. We've got four candidates:
> > > - max_sg_nents_burst
> > > - max_sg_burst
> > > - max_sg_chain
> > > - sg_chunk_len
> > > 
> > > @Vinod, @Andy, what do you think?
> > 
> 
> > So IIUC your hw supports single sg and in that you would like to publish
> > the length of each chunk, is that correct?
> 
> No. My DMA engine does support only a single-entry SG-list, but the new DMA
> {~~slave~~,channel,device,peripheral,...} capability isn't about the length, but
> is about the maximum number of SG-list entries a DMA engine is able to
> automatically/"without software help" walk through and execute. In this thread
> we are debating about that new capability naming.
> 
> The name suggested in this patch: max_sg_nents. Peter noted (I mostly agree with
> him), that it might be ambiguous, since from it (without looking into the
> dma_slave_caps structure comment) a user might think that it's a maximum number of
> SG-entries, which can be submitted for the DMA engine execution, while in fact it's
> about the DMA engine capability of automatic/burst/"without software intervention"
> SG-list entries walking through. (Such information will be helpful to solve a
> problem discussed in this mailing thread, and described in the cover-letter to
> this patchset. We also discussed it with you and Andy in the framework of this
> patchset many times.)
> 
> As an alternative Peter suggested: max_sg_nents_burst. I also think it's better
> than "max_sg_nents" but for me it seems a bit long. max_sg_burst seems better.
> There is no need in having the "nents" in the name, since SG-list implies a list,
> which main parameter (if not to say only parameter) is the number of entries.
> "burst" is pointing out to the automatic/accelerated/"without software intervention"
> SG-list entries walking through.
> 
> On the second thought suggested by me "max_sg_chain" sounds worse than "max_sg_burst",
> because it also might be perceived as a parameter limiting the number of SG-list
> entries is able to be submitted for the DMA engine execution, while in fact it
> describes another matter.
> 
> Regarding "sg_chunk_len". I think it's ambiguous too, since the "chunk
> length" might be referred to both the entries length and to the sub-SG-list
> length.
> 
> So what do you think? What name is better describing the new DMA capability?

How about max_nents_per_sg or max_nents_sg to signify that this implies
max nents for sg not sg entries. IMO Burst/chain are not better than
max_sg_nents.

-- 
~Vinod