linux-kernel - Re: [PATCH v2 0/4] Introduce the Counter character device interface

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Message-ID: <20200602154626.GA5082@shinobu>
Date:   Tue, 2 Jun 2020 11:46:26 -0400
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     David Lechner <david@...hnology.com>
Cc:     Jonathan Cameron <jic23@...nel.org>, kamel.bouhara@...tlin.com,
        gwendal@...omium.org, alexandre.belloni@...tlin.com,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, syednwaris@...il.com,
        patrick.havelange@...ensium.com, fabrice.gasnier@...com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com
Subject: Re: [PATCH v2 0/4] Introduce the Counter character device interface

On Tue, Jun 02, 2020 at 10:18:07AM -0500, David Lechner wrote:
> On 5/31/20 12:14 PM, William Breathitt Gray wrote:
> > Yielding the following /dev/counter0 memory layout:
> > 
> > +------------+-----------------+------------+-------------------+
> > | Byte 0     | Byte 1 - Byte 8 | Byte 9     | Byte 10 - Byte 17 |
> > +------------+-----------------+------------+-------------------+
> > | Boundary 0 | Count 0         | Boundary 1 | Count 1           |
> > +------------+-----------------+------------+-------------------+
> 
> A potential pitfall with this sort of packing is that some platforms
> do not support unaligned access, so data would have to be "unpacked"
> before it could be used.

Since the user defines the format of this data, they could reorganize it
to a more streamline alignment; for example:

# echo "C0 C1 C0E0 C1E0" > counter0/chrdev_format

Yielding the following /dev/counter0 memory layout instead:

+-----------------+------------------+------------+------------+
| Byte 0 - Byte 7 | Byte 8 - Byte 15 | Byte 16    | Byte 17    |
+-----------------+------------------+------------+------------+
| Count 0         | Count 1          | Boundary 0 | Boundary 1 |
+-----------------+------------------+------------+------------+

In the future, we could also define a padding argument to give users
more control over the exact offsets:

# echo "C0E0 P7 C0 C1E0 P7 C1" > counter0/chrdev_format

Yielding the following /dev/counter0 memory layout instead:

+------------+-----------------+------------------+------------+
| Byte 0     | Byte 1 - Byte 7 | Byte 8 - Byte 15 | Byte 16    |
+------------+-----------------+------------------+------------+
| Boundary 0 | Padding         | Count 0          | Boundary 1 |
+------------+-----------------+------------------+------------+
+-------------------+-------------------+
| Byte 17 - Byte 23 | Byte 24 - Byte 31 |
+-------------------+-------------------+
| Padding           | Count 1           |
+-------------------+-------------------+

I not sure it's best to introduce padding support with this patchset
given how much is already changing, but I don't anticipate packing
alignment to be something difficult to support in the future with this
interface.

William Breathitt Gray

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