linux-kernel - Re: [PATCH] spi: Add FSI-attached SPI controller driver

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Date:   Wed, 5 Feb 2020 17:51:52 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Eddie James <eajames@...ux.ibm.com>
Cc:     Eddie James <eajames@...ux.vnet.ibm.com>,
        linux-spi <linux-spi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>, Joel Stanley <joel@....id.au>,
        Andrew Jeffery <andrew@...id.au>
Subject: Re: [PATCH] spi: Add FSI-attached SPI controller driver

On Tue, Feb 4, 2020 at 6:06 PM Eddie James <eajames@...ux.ibm.com> wrote:
> On 2/4/20 5:02 AM, Andy Shevchenko wrote:
> > On Mon, Feb 3, 2020 at 10:33 PM Eddie James <eajames@...ux.vnet.ibm.com> wrote:
> >> On 1/30/20 10:37 AM, Andy Shevchenko wrote:
> >>> On Wed, Jan 29, 2020 at 10:09 PM Eddie James <eajames@...ux.ibm.com> wrote:

...

> >>>> +       struct device *dev;
> >>> Isn't fsl->dev the same?
> >>> Perhaps kernel doc to explain the difference?
> >>
> >> No, it's not the same, as dev here is the SPI controller. I'll add a
> >> comment.
> > Why to have duplication then?
>
>
> Nothing is being duplicated, the two variables are storing entirely
> different information, both of which are necessary for each SPI
> controller that this driver is driving.

Oh, I see now, thanks!

...

> >>>> +       for (i = 0; i < num_bytes; ++i)
> >>>> +               rx[i] = (u8)((in >> (8 * ((num_bytes - 1) - i))) & 0xffULL);
> >>> Redundant & 0xffULL part.
> >>>
> >>> Isn't it NIH of get_unalinged_be64 / le64 or something similar?
> >>
> >> No, these are shift in/out operations. The read register will also have
> >> previous operations data in them and must be extracted with only the
> >> correct number of bytes.
> > Why not to call put_unaligned() how the tail in this case (it's 0 or
> > can be easily made to be 0) will affect the result?
>
>
> The shift-in is not the same as any byte-swap or unaligned operation.
> For however many bytes we've read, we start at that many bytes
> left-shifted in the register and copy out to our buffer, moving right
> for each next byte... I don't think there is an existing function for
> this operation.

For me it looks like

  u8 tmp[8];

  put_unaligned_be64(in, tmp);
  memcpy(rx, tmp, num_bytes);

put_unaligned*() is just a method to unroll the value to the u8 buffer.
See, for example, linux/unaligned/be_byteshift.h implementation.

> >>>> +       return num_bytes;
> >>>> +}
> >>>> +static int fsi_spi_data_out(u64 *out, const u8 *tx, int len)
> >>>> +{
> >>> Ditto as for above function. (put_unaligned ...)
> > Ditto.
>
>
> I don't understand how this could work for transfers of less than 8
> bytes, any put_unaligned would access memory that it doesn't own.

Ditto.

> >>>> +}

-- 
With Best Regards,
Andy Shevchenko