linux-kernel - Re: [PATCH] USB: OHCI: ohci-sm501: complete URBs in BH context

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Date:   Mon, 6 Aug 2018 01:37:30 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: OHCI: ohci-sm501: complete URBs in BH context

On Fri, Aug 03, 2018 at 09:01:19PM -0700, Guenter Roeck wrote:
> Testing an USB drive connected to ohci-sm501 results in a large number
> of runtime warnings.

As far as I can tell this driver uses the HCD_LOCAL_MEM feature flag
for memory declared using dma_declare_coherent_memory.  Unlike the
mormal dma mapping interfaces this special case can actually be freed
from interrupt context, and we have a fix for this warning queued
up in linux-next:

http://git.infradead.org/users/hch/dma-mapping.git/commitdiff/d27fb99f62af7b79c542d161aa5155ed57271ddc

That being said I'm generally very unhappy how dma_init_coherent_memory
turned out.  The idea was to allow device local memory to be hidden
behind the DMA API, but in general we use it either as a way to declare
special uncache system memory (which would really be the plaform codes
job), or as a magic bounce buffer like in the USB code.  I plan to
eventually untangle this, but it is going to take some time.