linux-kernel - Re: [PATCH v2 2/6] dt-bindings: dma: dw: Add max burst transaction length property

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Date:   Tue, 12 May 2020 00:35:31 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Vinod Koul <vkoul@...nel.org>,
        Viresh Kumar <vireshk@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-mips@...r.kernel.org, dmaengine@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/6] dt-bindings: dma: dw: Add max burst transaction
 length property

On Tue, May 12, 2020 at 12:01:38AM +0300, Andy Shevchenko wrote:
> On Mon, May 11, 2020 at 11:05:28PM +0300, Serge Semin wrote:
> > On Fri, May 08, 2020 at 02:12:42PM +0300, Andy Shevchenko wrote:
> > > On Fri, May 08, 2020 at 01:53:00PM +0300, Serge Semin wrote:
> > > > This array property is used to indicate the maximum burst transaction
> > > > length supported by each DMA channel.
> > > 
> > > > +  snps,max-burst-len:
> > > > +    $ref: /schemas/types.yaml#/definitions/uint32-array
> > > > +    description: |
> > > > +      Maximum length of burst transactions supported by hardware.
> > > > +      It's an array property with one cell per channel in units of
> > > > +      CTLx register SRC_TR_WIDTH/DST_TR_WIDTH (data-width) field.
> > > > +    items:
> > > > +      maxItems: 8
> > > > +      items:
> > > 
> > > > +        enum: [4, 8, 16, 32, 64, 128, 256]
> > > 
> > > Isn't 1 allowed?
> > 
> > Burst length of 1 unit is supported, but in accordance with Data Book the MAX
> > burst length is limited to be equal to a value from the set I submitted. So the
> > max value can be either 4, or 8, or 16 and so on.
> 
> Hmm... It seems you mistakenly took here DMAH_CHx_MAX_MULT_SIZE pre-silicon
> configuration parameter instead of runtime as described in Table 26:
> CTLx.SRC_MSIZE and DEST_MSIZE Decoding.

No. You misunderstood what I meant. We shouldn't use a runtime parameters values
here. Why would we? Property "snps,max-burst-len" matches DMAH_CHx_MAX_MULT_SIZE
config parameter. See a comment to the "SRC_MSIZE" and "DEST_MSIZE" fields of the
registers. You'll find out that their maximum value is determined by the
DMAH_CHx_MAX_MULT_SIZE parameter, which must belong to the set [4, 8, 16, 32, 64,
128, 256]. So no matter how you synthesize the DW DMAC block you'll have at least
4x max burst length supported.

-Sergey

> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>