Date: Tue, 10 Jan 2012 12:54:36 -0800 (PST) From: Matthew Jacob <mj@feral.com> To: Andriy Gapon <avg@freebsd.org> Cc: FreeBSD current <freebsd-current@freebsd.org> Subject: Re: bus dma: a flag/quirk for page zero Message-ID: <alpine.BSF.2.00.1201101252530.86261@ns1.feral.com> In-Reply-To: <4F0C9D14.60705@FreeBSD.org> References: <4F0C9D14.60705@FreeBSD.org>
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I think it would be just simpler to disallow page zero usage period. Can you think of any case where physical page 0 is ever a valid DMA address? At the very least, require bounce buffers. On Tue, 10 Jan 2012, Andriy Gapon wrote: > > > Some hardware interfaces may reserve a special meaning for a (physical) memory > address value of zero. One example is the OHCI specification where a zero value > in CurrentBufferPointer doesn't mean a physical address, but has a reserved > meaning. To be honest I don't have another example :) but don't preclude its > existence. > > To deal with this peculiarity we could use a special flag/quirk that would > instruct the bus dma code to never use the page zero for communication with the > hardware. > Here's a proof of concept patch that implements the idea: > http://people.freebsd.org/~avg/usb-dma-pagezero.diff > > Some concerns: > - not sure if BUS_DMA_NO_PAGEZERO is the best name for the flag > - the patch implements the flag only for x86 at the moment > - usb code uses the flag regardless of the actual controller type > > What do you think? > > -- > Andriy Gapon > _______________________________________________ > freebsd-current@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to "freebsd-current-unsubscribe@freebsd.org" >
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