linux-kernel - Re: [Patch] Increase USBFS Bulk Transfer size

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Date:	Fri, 14 Oct 2011 09:51:26 +0100
From:	James Courtier-Dutton <james.dutton@...il.com>
To:	Markus Rechberger <mrechberger@...il.com>
Cc:	Valdis.Kletnieks@...edu, USB list <linux-usb@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, Greg KH <gregkh@...e.de>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [Patch] Increase USBFS Bulk Transfer size

On 14 October 2011 07:23, Markus Rechberger <mrechberger@...il.com> wrote:
> On Fri, Oct 14, 2011 at 7:47 AM,  <Valdis.Kletnieks@...edu> wrote:
>> On Fri, 14 Oct 2011 05:42:44 +0200, Markus Rechberger said:
>>
>>> The inflexible device which uses 24064 bytes works with all other
>>> Operating systems by using that value
>>> and gives exactly the same results with other transfer sizes than that.
>>
>> -ENOPARSE.  If it's inflexible,
>
> the particular device in question is inflexible yes.
>
Here is what I think the actual situation is.
Your transfers over the USB are done in 512 Bytes chunks.
If the device wishes to send 512 bytes, it sends one chunk.
If the device wishes to send 513 bytes, it sends only one chunk,
missing one byte.
if the device wishes to send 1023 bytes, it sends only one chunk,
missing 511 bytes.
if the device wishes to send 1024 bytes, it sends two chunks, missing 0 bytes.
So, the device is sending 1 too few chunks unless the bytes size
exactly matches the chunk size * n.
Another constraint is the device sends multiples of 188 bytes.
So, unless we can find a lowest common multiple of 188 and 512, there
is no transfer size that will work with this device. The LCM is 24064
and this is the only value that will work with this device.

Conclusion:
The hardware on the Linux PC and the kernel on the Linux PC are
working correctly.
Your external USB device has an off-by-one error in its
hardware/embedded software and the hardware/embedded software
manufacturer is not prepared to fix it.

Workaround in software: Increase MAX from 16384 to 24064 or above.

My own feeling, throw away this faulty bit of hardware and use a different part.
The hardware is not compatible with USB standards.

Kind Regards

James
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