linux-kernel - Re: [PATCH 2/3] HID: steam: add serial number information.

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Message-ID: <20180215221633.GA18755@casa>
Date:   Thu, 15 Feb 2018 23:16:33 +0100
From:   Rodrigo Rivas Costa <rodrigorivascosta@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Jiri Kosina <jikos@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>, linux-input@...r.kernel.org
Subject: Re: [PATCH 2/3] HID: steam: add serial number information.

On Wed, Feb 14, 2018 at 03:51:31PM +0100, Benjamin Tissoires wrote:
> On Tue, Feb 13, 2018 at 1:03 PM, Rodrigo Rivas Costa
> > +#define STEAM_FEATURE_REPORT_SIZE 65
> > +
> > +static int steam_send_report(struct steam_device *steam,
> > +               u8 *cmd, int size)
> > +{
> > +       int retry;
> > +       int ret;
> > +       u8 *buf = kzalloc(STEAM_FEATURE_REPORT_SIZE, GFP_KERNEL);
> 
> Please use hid_alloc_report_buf() as sometimes we need to allocate a
> slightly bigger report.

I have an issue with this one. The problem is that using
hid_report_len() on the feature report returns 64. But I must call
hid_hw_raw_request() with 65 or it will fail with EOVERFLOW.

Currently I'm allocating a buffer of 65 bytes and all is well.
If I change to hid_alloc_report_buf(), the current implementation
allocates (64+7), so I'm still safe. But I'm worried that the extra
bytes are not guaranteed and a future implementation could return
exactly 64 bytes, leaving me 1 byte short.

About why an array of 65 is required for a report of size 64, I think it
is related to hid_report->id == 0 (so hid_report_enum->numbered == 0).

So what would be the proper solution?

Thanks.
Rodrigo.