Re: [PATCH v3 RESEND] dma-mapping: add new API formax_segment_number

From: Matthew Wilcox
Date: Fri Jun 17 2011 - 08:40:49 EST

On Thu, Jun 16, 2011 at 08:30:53PM +0800, Shawn Guo wrote:
> Here is the user story that tells the need of the new api. The
> mxs-mmc is the mmc host controller for Freescale MXS architecture.
> There are a pair of mmc host specific parameters max_seg_size and
> max_segs that mxs-mmc host driver needs to tell mmc core, so that
> mmc core can know how big each data segment could be and how many
> segments could be handled one time in a scatter list by host driver.
> The mxs-mmc driver is one user of dmaengine mxs-dma, and it will call
> mxs-dma to transfer data in scatter list. That is to say mxs-mmc has
> no idea of what max_seg_size and max_segs should be, because they are
> all mxs-dma capability parameters, and mxs-mmc needs to query them
> from mxs-dma.

This approach would make sense if mxs-mmc were generic, but it's tied to
mxs-dma, so it can just as well call mxs-dma to find out how many segments
it supports.

> Right now, there is well defined dma api (dma_get_max_seg_size) for
> mmc to query max_seg_size from dma driver, but the one for max_segs
> is missing. That's why mxs-mmc driver has to hard-code it.
> The mxs-mmc is just one example to demonstrate the need of the new
> api, and there are other mmc host drivers (mxcmmc on imx-dma is
> another example) and possibly even other dmaengine users need this
> new api to know the maximum segments that dma driver can handle per
> dma call.

Again, mxcmmc can just call imx-dma directly.

Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
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