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Date:   Tue, 21 Jun 2022 11:11:53 -0400
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Marc Kleine-Budde <mkl@pengutronix.de>,
        Rhett Aultman <rhett.aultman@samsara.com>,
        linux-usb@vger.kernel.org, linux-can <linux-can@vger.kernel.org>,
        Oliver Neukum <oneukum@suse.com>
Subject: Re: [PATCH v3 1/2] drivers: usb/core/urb: Add URB_FREE_COHERENT
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On Tue, Jun 21, 2022 at 11:59:16PM +0900, Vincent MAILHOL wrote:
> I (probably wrongly) assumed that urb::transfer_buffer_length was the
> allocated length and urb::actual_length was what was actually being
> transferred. Right now, I am just confused. Seems that I need to study
> a bit more and understand the real purpose of
> urb::transfer_buffer_length because I still fail to understand in
> which situation this can be different from the allocated length.

urb->transfer_buffer_length is the amount of data that the driver wants 
to send or expects to receive.  urb->actual_length is the amount of data 
that was actually sent or actually received.

Neither of these values has to be the same as the size of the buffer -- 
but they better not be bigger!

Alan Stern