linux-kernel - Re: [PATCH 2/4] usbnet: smsc95xx: align tx-buffer to word

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Message-ID: <1538671996.3024.269.camel@codethink.co.uk>
Date:   Thu, 04 Oct 2018 17:53:16 +0100
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Ben Dooks <ben.dooks@...ethink.co.uk>, netdev@...r.kernel.org
Cc:     linux-kernel@...ts.codethink.co.uk, linux-usb@...r.kernel.org,
        oneukum@...e.com, linux-kernel@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH 2/4] usbnet: smsc95xx: align tx-buffer to
 word

On Tue, 2018-10-02 at 10:26 +0100, Ben Dooks wrote:
> The tegra driver requires alignment of the buffer, so try and
> make this better by pushing the buffer start back to an word
> aligned address. At the worst this makes memcpy() easier as
> it is word aligned, at best it makes sure the usb can directly
> map the buffer.
> 
> Signed-off-by: Ben Dooks <ben.dooks@...ethink.co.uk>
> [todo - make this configurable]
[...]

I don't think you need a separate kconfig symbol for this.  Aligning RX
buffers to words (or better, cache lines) is almost always a win, so
long as the CPU can handle misaligned fields in the network/transport
headers.  You can use #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to
check that.

It seems like NET_IP_ALIGN should be defined to 0 or 2 depending on
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, but NET_IP_ALIGN predates the
latter.

Ben.

-- 
Ben Hutchings, Software Developer                         Codethink Ltd
https://www.codethink.co.uk/                 Dale House, 35 Dale Street
                                     Manchester, M1 2HF, United Kingdom