Date: Tue, 23 Jun 2009 10:35:42 +0200 From: Piotr =?iso-8859-2?q?Zi=EAcik?= <kosmo@semihalf.com> To: Hans Petter Selasky <hselasky@c2i.net> Cc: Rafal Jaworowski <raj@semihalf.com>, thompsa@FreeBSD.org, freebsd-usb@freebsd.org Subject: CPU Cache and busdma usage in USB Message-ID: <200906231035.43096.kosmo@semihalf.com>
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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
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