CPU Cache and busdma usage in USB

[Alexandr Rybalko]

On Tue, 23 Jun 2009 10:35:42 +0200
Piotr Zięcik <kosmo at semihalf.com> wrote:

>> While bringing up EHCI  (8-CURRENT, new USB stack) on ARM machine we have 
>> found cache-related problem in the USB stack.
>> 
>> The usb_pc_cpu_flush() and usb_pc_cpu_invalidate() functions are used to 
>> flush/invalidate CPU caches in various places in USB code. Internally, the 
>> functions are implemented using bus_dmamap_sync(). In our case, on ARM 
>> machine, flags passed to the bus_dmamap_sync() function did not
>> correspond with requested operation. We have fixed the problem by changing 
>> flags passed to the bus_dmamap_sync() function (see attached patch).
>> 
>> My question is about general idea of bus_dma usage for cache operations. In my 
>> opinion we should not rely on bus_dmamap_sync() behaviour as this function 
>> may do different things on different architectures.  This not always works as 
>> expected, which is clearly visible in our case. Better solution is to use 
>> cpu-specific functions implementing cache operations. Please comment on why 
>> CPU-specific functions are not used...
>> 
>> Patch fixing our problem:
>> diff --git a/sys/dev/usb/usb_busdma.c b/sys/dev/usb/usb_busdma.c
>> index 3d6a5be..69a6fff 100644
>> --- a/sys/dev/usb/usb_busdma.c
>> +++ b/sys/dev/usb/usb_busdma.c
>> @@ -658,8 +658,7 @@ usb_pc_cpu_invalidate(struct usb_page_cache *pc)
>>                 /* nothing has been loaded into this page cache! */
>>                 return;
>>         }
>> -       bus_dmamap_sync(pc->tag, pc->map,
>> -           BUS_DMASYNC_POSTWRITE | BUS_DMASYNC_POSTREAD);
>> +       bus_dmamap_sync(pc->tag, pc->map, BUS_DMASYNC_PREREAD);
>>  }
>> 
>>  /*------------------------------------------------------------------------*
>> @@ -672,8 +671,7 @@ usb_pc_cpu_flush(struct usb_page_cache *pc)
>>                 /* nothing has been loaded into this page cache! */
>>                 return;
>>         }
>> -       bus_dmamap_sync(pc->tag, pc->map,
>> -           BUS_DMASYNC_PREWRITE | BUS_DMASYNC_PREREAD);
>> +       bus_dmamap_sync(pc->tag, pc->map, BUS_DMASYNC_PREWRITE);
>>  }
>> 
>>  /*------------------------------------------------------------------------*
>> 
>> -- 
>> Best regards.
>> Piotr Ziecik
>> _______________________________________________
>> freebsd-usb at freebsd.org mailing list
>> http://lists.freebsd.org/mailman/listinfo/freebsd-usb
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Great thanks Piotr!
I work on MIPS BCM5354 and BCM5836 and after apply your patch USB work correct.



-- 
Alexandr Rybalko <ray at dlink.ua>