linux-kernel - Re: [PATCH 2/2] can: esd_usb: Add support for esd CAN-USB/3

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Message-ID: <20230509-smirk-viewing-5e13ea0abfeb-mkl@pengutronix.de>
Date:   Tue, 9 May 2023 09:06:04 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>
Cc:     Frank Jungclaus <Frank.Jungclaus@....eu>,
        Stefan Mätje <Stefan.Maetje@....eu>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "wg@...ndegger.com" <wg@...ndegger.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] can: esd_usb: Add support for esd CAN-USB/3

On 09.05.2023 10:28:13, Vincent MAILHOL wrote:
> > > And because hdr.len is initially 3, hdr.len becomes 5. Right? Shouldn't it be 8?
> >
> > It might be a little confusing, but I think it's fine.
> > hdr.len is given in units of longwords (4 bytes each)! Therefore we
> > have 12 bytes (the initial 3 longwords) for struct tx_msg before
> > tx_msg.data[].
> > Than (8 + 3)/4=2 gives us 2 additional longwords for the 8 data bytes.
> > So that 3+2=5 (equal to 20 bytes) should be ok.

I think the term longword is more commonly used in non-Unix operating
systems :)

> OK. So you want to round up the length to the next sizeof(long) multiple, right?
> 
> First, sizeof(long) being platform specific, you need to declare a
> macro to make your intent explicit.
> 
> /* Size of a long int on esd devices */
> #define USB_ESD_SIZEOF_LONG 4
> 
> Please test, but for what I understand, below line is an equivalent
> and a more readable way to achieve your goal:
> 
>           msg->hdr.len = DIV_ROUND_UP(cf->len, USB_ESD_SIZEOF_LONG);

use "sizeof(u32)"

> Also, add documentation to your structure to explain that hdr.len
> represents the length in long, not in bytes.

...lengths in multiple of u32.

Marc

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