Re: [PATCH] hw/usb/hcd-ohci: Fix ohci_service_td: accept valid TDs

["Alex Bennée" <alex.bennee@linaro.org>]

David Hubbard <dmamfmgm@gmail.com> writes:

> From: Cord Amfmgm <dmamfmgm@gmail.com>
>
> This changes the way the ohci emulation handles a Transfer Descriptor with
> "Current Buffer Pointer" set to "Buffer End" + 1.
>
> The OHCI spec 4.3.1.2 Table 4-2 allows td.cbp to be one byte more than td.be
> to signal the buffer has zero length. Currently qemu only accepts zero-length
> Transfer Descriptors if the td.cbp is equal to 0, while actual OHCI hardware
> accepts both cases.

Which version of the OHCI spec is this? I can't find it in the one copy
Google throws up:

  http://download.microsoft.com/download/1/6/1/161ba512-40e2-4cc9-843a-923143f3456c/ohci_11.pdf

> The qemu ohci emulation has a regression in ohci_service_td. Version 4.2
> and earlier matched the spec. (I haven't taken the time to bisect exactly
> where the logic was changed.)
>
> With a tiny OS[1] that boots and executes a test, the issue can be seen:
>
> * OS that sends USB requests to a USB mass storage device
>   but sends td.cbp = td.be + 1
> * qemu 4.2
> * qemu HEAD (4e66a0854)
> * Actual OHCI controller (hardware)
<snip>

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro