linux-kernel - RE: [PATCH] mtd: spi-nor: fsl-quadspi: add big-endian support

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Message-ID: <BLUPR03MB134D5B98BF8C5DA5E763B1E831E0@BLUPR03MB134.namprd03.prod.outlook.com>
Date:	Mon, 16 Nov 2015 04:08:39 +0000
From:	Yao Yuan <yao.yuan@...escale.com>
To:	Brian Norris <computersforpeace@...il.com>
CC:	Fabio Estevam <festevam@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Li Leo <LeoLi@...escale.com>,
	Scott Wood <scottwood@...escale.com>,
	Xu Han <han.xu@...escale.com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mark Brown <broonie@...nel.org>
Subject: RE: [PATCH] mtd: spi-nor: fsl-quadspi: add big-endian support

Hi Brian Norris,

Thanks for your information and the documents shared by you.
It's very helpful for me to understand the regmap.

But I think if we use:
static void qspi_writel(struct fsl_qspi *q, u32 val, void __iomem
*addr) {
       if (q->big_endian)
               iowrite32be(val, addr);
       else
               iowrite32(val, addr);
 }
It looks like easier to read, use and change.
Is it?

And David Woodhouse, Xu Han, Mark Brown
is there any other comments from you?

Thank.

On Thu, Nov 12, 2015 3:04 AM Brian Norris wrote:
> On Fri, Oct 30, 2015 at 09:49:41AM +0000, Yao Yuan wrote:
> > Hi Fabio Estevam,
> >
> > Thanks for your suggestion.
> > We have an internal discussions for that.
> >
> > We think that:
> > According to the initial commit message of regmap, it is targeting
> > non-memory mapped buses. (regmap: Add generic non-memory mapped
> > register access API)  But in the imx2_wdt driver, it is used for
> > memory-mapped register space.  So it seems that using such a complex
> > framework just to deal with endian is an over-kill.
> 
> It is definitely useful for non-MMIO cases, but it's certainly not exclusive too it.
> 
> > when it is not necessary to enable the clock every time we access the register.
> 
> You don't have to give it a clock. Just pass a NULL clk_id.
> 
> > We don't think it is obvious to us how to use it for handling
> > endianness, especially not the way imx2_wdt uses regmap.
> > __regmap_init_mmio_clk() calls regmap_mmio_gen_context() which errors
> > out if reg_format_endian is not REGMAP_ENDIAN_DEFAULT or
> > REGMAP_ENDIAN_NATIVE, and elsewhere regmap-mmio.c It seems only
> > little-endian accessors.
> >
> > Although it is possible to add the endianness support in the
> > regmap_mmio driver, we don't see too much value in using it especially
> 
> It already has DT endianness configuration support. See __regmap_init(),
> which reconfigures the endianness according to regmap_get_val_endian().
> So you don't need to do anything but just try it... I exepct it'll work just fine.
> 
> > So we think:
> > static void qspi_writel(struct fsl_qspi *q, u32 val, void __iomem
> > *addr) {
> >       if (q->big_endian)
> >               iowrite32be(val, addr);
> >       else
> >               iowrite32(val, addr);
> > }
> > This way is an easier, more effective solution to do the endian issue.
> >
> > How about your think?
> 
> I think there's at least one more advantage: you get pretty good tracing
> support for free. For debugging, for example, you can turn on regmap tracing
> to see all the register reads and writes done in your driver, all within the nice
> tracefs event infrastructure. I'm sure there are other advantages too, but not
> all are applicable here.
> 
> Anyway, I do agree on the complexity argument. It's not actually that complex
> to use (the imx2_wdt.c example really does show you everything you need to
> know), it is a bit more complex to sort through all its features and understand
> exactly what it's doing. But I'm confident it has everything you need.
> 
> So, make your choice. I just wanted to educate on several points, so that your
> decision is not just driven by lack of correct information.
> 
> For more information, a quick google search shows a few links, but no official
> docs:
> 
> http://elinux.org/images/a/a3/Regmap-
> _The_Power_of_Subsystems_and_Abstractions.pdf
> https://lwn.net/Articles/451789/
> 
> Brian
> 
> > Best Regards,
> > Yuan Yao
> >
> > On Sat, Oct 24, 2015 at 11:47 PM, Fabio Estevam <festevam@...il.com>
> wrote:
> > > On Fri, Oct 23, 2015 at 5:53 AM, Yuan Yao <yao.yuan@...escale.com> wrote:
> > >
> > > > +static void qspi_writel(struct fsl_qspi *q, u32 val, void __iomem
> > > > +*addr) {
> > > > +       if (q->big_endian)
> > > > +               iowrite32be(val, addr);
> > > > +       else
> > > > +               iowrite32(val, addr); }
> > >
> > > I suggest you to implement regmap support for this driver instead.
> > >
> > > Take a look at drivers/watchdog/imx2_wdt.c for a reference.
> > >
> > > Then you only need to pass 'big-endian' as a property for the qspi
> > > in the .dtsi file and regmap core will take care of endianness.
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