Re: [RFC PATCH 04/12] s390/cio: introduce cio DMA pool

[Sebastian Ott <sebott@linux.ibm.com>]

On Fri, 12 Apr 2019, Halil Pasic wrote:
> On Thu, 11 Apr 2019 20:25:01 +0200 (CEST)
> Sebastian Ott <sebott@linux.ibm.com> wrote:
> > I don't think we should use this global DMA pool. I guess it's OK for
> > stuff like airq (where we don't have a struct device at hand) but for
> > CCW we should use the device we have. Yes, this way we waste some memory
> > but all dma memory a device uses should fit in a page - so the wastage
> > is not too much.
> > 
> 
> Is what you envision an own gen_pool on for each subchannel (e.g. a
> struct io_subchannel member)?

Either that or if that's too much overhead simply map a page and create
a struct containing the few dma areas for that device.

> I'm struggling with understanding the expected benefits of a
> per-subchannel pool/allocator. Can you please tell me what benefits do
> you expect (over the current approach)?

Logically DMA is a capability of a device and the whole DMA API is build
around devices. Working around that just feels wrong. For practical
matters: DMA debugging will complain about misuse of a specific device or
driver.

> I understand you idea is to keep the CIO global pool for stuff that can
> not be tied to a single device (i.e. ariq). So the per device stuff would
> also mean more code. Would you be OK with postponing this alleged
> enhancement (i.e. implement it as a patch on top of this series)?

I don't like it but it's just in-kernel usage which we can change at any
time. So if it helps you to do it that way, why not..