Date: Sun, 28 Jun 2009 11:54:40 +0200 From: Hans Petter Selasky <hselasky@c2i.net> To: Alexandr Rybalko <ray@dlink.ua>, freebsd-arm@freebsd.org Cc: freebsd-usb@freebsd.org, Piotr =?utf-8?q?Zi=C4=99cik?= <kosmo@semihalf.com>, thompsa@freebsd.org Subject: Re: CPU Cache and busdma usage in USB Message-ID: <200906281154.43392.hselasky@c2i.net> In-Reply-To: <200906231912.20741.hselasky@c2i.net> References: <200906231035.43096.kosmo@semihalf.com> <20090623121129.d18492a3.ray@dlink.ua> <200906231912.20741.hselasky@c2i.net>
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Hi Piotr and Rafal,
Your patch is not fully correct. It will break support for x86 and more when
bounce pages are uses.
Let's get the definitions right:
man busdma
<cite>
BUS_DMASYNC_PREREAD Perform any synchronization required prior
to an update of host memory by the device.
BUS_DMASYNC_PREWRITE Perform any synchronization required after
an update of host memory by the CPU and
prior to device access to host memory.
BUS_DMASYNC_POSTREAD Perform any synchronization required after
an update of host memory by the device and
prior to CPU access to host memory.
BUS_DMASYNC_POSTWRITE Perform any synchronization required after
device access to host memory.
</cite>
My view:
XXX_PREXXX functions should be used prior to read/write device access.
In other words, PRE has to be a flush operation.
XXX_POSTXXX functions should be used after read/write device access.
In other words, POST has to be an invalidate operation.
Reading:
src/sys/arm/arm/busdma_machdep.c
I find bus_dmamap_sync_buf() to be coherent with this view.
Can you check if the COHERENT bit is set for your allocation?
if (map->flags & DMAMAP_COHERENT)
return;
Summed up:
The existing code is doing correct. What is known is a problem with the memory
mapping, so that the same memory page can get mapped with different
attributes, which makes the problem appear.
--HPS
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