Re: PCI DMA mapping question

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-----Original Message-----
From: Momchil Velikov [mailto:velco@fadata.bg]
Sent: Thursday, March 25, 2004 3:19 PM
To: Mandeep Singh Sandhu
Subject: Re: PCI DMA mapping question


>>>>> "Mandeep" == Mandeep Singh Sandhu <mandeep_sandhu@infosys.com> writes:

Mandeep> if i'm allocating by transmit and recieve buffers,
Mandeep> for my ethernet driver, using pci_alloc_consistent
Mandeep> then do i need to establish bounce buffers as well
Mandeep> in case i get a mapping in a non-DMA zone??
Mandeep> but in Rubini's book it's written - "Allocation is
Mandeep> handled in this function so that the buffer will be
Mandeep> placed in a location that works with DMA..."

  That function checks the device addressing capabilities.  It assumes
that if a device can be addressed in a region with certain
characteristics then *IT* can also address any region with the same
characteristics.  For example, if a device requested to be accessible
for MMIO in a region under 1M, the kernel assumes that the device
itself can address any region under 1M, thus it allocates page(s) with
GFP_DMA set.

Mandeep> so then do i need to keep bounce buffers??

  I'm not sure what's the exact meaning of ``bounce buffer''.  I tend
to think that these ARE the bounce buffers (as opposed to having
potentially larger buffers elsewhere or locking user pages for doing
DMA to them).

Mandeep> then it says that the allocation uses GFP_ATOMIC
Mandeep> priority, shud'nt it use the GFP_DMA priority instead
Mandeep> to ensure the addresses are allocated in the DMA zone??

  GFP_ATOMIC and GPF_DMA are independent and can be set at once.
GFP_ATOMIC means ``don't sleep'' (which, btw, is by no means
``atomic''), GPF_DMA means ``allocate under 1M''.

~velco

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