linux-kernel - RE: [PATCH 07/10] spi: rzv2m-csi: Switch to using {read,write}s{b,w}

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Date:   Mon, 17 Jul 2023 13:00:21 +0000
From:   Fabrizio Castro <fabrizio.castro.jz@...esas.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Mark Brown <broonie@...nel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Chris Paterson <Chris.Paterson2@...esas.com>,
        Biju Das <biju.das@...renesas.com>,
        Prabhakar Mahadev Lad <prabhakar.mahadev-lad.rj@...renesas.com>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>
Subject: RE: [PATCH 07/10] spi: rzv2m-csi: Switch to using {read,write}s{b,w}

Hi Andy,

Thanks for your reply.

> From: Andy Shevchenko <andy.shevchenko@...il.com>
> Subject: Re: [PATCH 07/10] spi: rzv2m-csi: Switch to using
> {read,write}s{b,w}
> 
> On Mon, Jul 17, 2023 at 1:36 PM Fabrizio Castro
> <fabrizio.castro.jz@...esas.com> wrote:
> > > From: Geert Uytterhoeven <geert@...ux-m68k.org>
> > > Subject: Re: [PATCH 07/10] spi: rzv2m-csi: Switch to using
> > > {read,write}s{b,w}
> > > On Sat, Jul 15, 2023 at 3:04 AM Fabrizio Castro
> > > <fabrizio.castro.jz@...esas.com> wrote:
> 
> ...
> 
> > > According to the hardware documentation[1], the access size for
> both
> > > the
> > > CSI_OFIFO and CSI_IFIFO registers is 32 bits, so you must use
> writel()
> > > resp. readl().  So please check with the hardware people first.
> > >
> > > [1] RZ/V2M User's Manual Hardware, Rev. 1.30.
> >
> > You are right, access is 32 bits (and although this patch works
> fine,
> > we should avoid accessing those regs any other way). Now I remember
> > why I decided to go for the bespoke loops in the first place,
> writesl
> > and readsl provide the right register access, but the wrong pointer
> > arithmetic for this use case.
> > For this patch I ended up selecting writesw/writesb/readsw/readsb to
> > get the right pointer arithmetic, but the register access is not as
> > per manual.
> >
> > I can either fallback to using the bespoke loops (I can still
> > remove the unnecessary u8* and u16* casting ;-) ), or I can add
> > new APIs for this sort of access to io.h (e.g. writesbl, writeswl,
> > readsbl, readswl, in order to get the pointer arithmetic right for
> > the type of array handled, while keeping memory access as expected).
> >
> > What are your thoughts on that?
> 
> I think that you need to use readsl() / writesl() on the custom buffer
> with something like
> 
> *_sparse() / *_condence() APIs added (perhaps locally to this driver)
> as they may be reused by others in the future.
> Having all flavours of read*()/write*() does not scale in my opinion.

Maybe a "generic" macro then (one for reading and one for writing)?
So that we can pass the data type (to get the pointer arithmetic
right) and the function name to use for the read/write operations
(to get the register operations right)?
Maybe that would scale and cater for most needs (including the one
from this patch)?

Cheers,
Fab

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