linux-kernel - Re: [PATCH v2,RESEND] misc: new driver sram

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Date:   Mon, 27 Apr 2020 17:54:53 -0500
From:   Scott Wood <oss@...error.net>
To:     Rob Herring <robh@...nel.org>, Wang Wenhu <wenhu.wang@...o.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>, kernel@...o.com,
        Christophe Leroy <christophe.leroy@....fr>,
        Michael Ellerman <mpe@...erman.id.au>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v2,RESEND] misc: new driver sram_uapi for user level
 SRAM access

On Mon, 2020-04-27 at 09:13 -0500, Rob Herring wrote:
> On Sun, Apr 19, 2020 at 10:06 PM Wang Wenhu <wenhu.wang@...o.com> wrote:
> > 
> > A generic User-Kernel interface that allows a misc device created
> > by it to support file-operations of ioctl and mmap to access SRAM
> > memory from user level. Different kinds of SRAM alloction and free
> > APIs could be registered by specific SRAM hardware level driver to
> > the available list and then be chosen by users to allocate and map
> > SRAM memory from user level.
> > 
> > It is extremely helpful for the user space applications that require
> > high performance memory accesses, such as embedded networking devices
> > that would process data in user space, and PowerPC e500 is a case.
> > 
> > Signed-off-by: Wang Wenhu <wenhu.wang@...o.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Christophe Leroy <christophe.leroy@....fr>
> > Cc: Scott Wood <oss@...error.net>
> > Cc: Michael Ellerman <mpe@...erman.id.au>
> > Cc: Randy Dunlap <rdunlap@...radead.org>
> > Cc: linuxppc-dev@...ts.ozlabs.org
> > ---
> > Changes since v1: addressed comments from Arnd
> >  * Changed the ioctl cmd definitions using _IO micros
> >  * Export interfaces for HW-SRAM drivers to register apis to available
> > list
> >  * Modified allocation alignment to PAGE_SIZE
> >  * Use phys_addr_t as type of SRAM resource size and offset
> >  * Support compat_ioctl
> >  * Misc device name:sram
> > 
> > Note: From this on, the SRAM_UAPI driver is independent to any hardware
> > drivers, so I would only commit the patch itself as v2, while the v1 of
> > it was wrapped together with patches for Freescale L2-Cache-SRAM device.
> > Then after, I'd create patches for Freescale L2-Cache-SRAM device as
> > another series.
> 
> There's work to add SRAM support to dma-buf heaps[1]. Take a look and
> see if that works for you.
> 
> Rob
> 
> [1] https://lore.kernel.org/lkml/20200424222740.16259-1-afd@ti.com/
> 

The dma heap API itself (what makes it specific to DMA, rather than any
special-purpose allocator?) seems like it could be what we're looking for. 
The issue with drivers/misc/sram.c is that it seems like its main purpose is
to get sram description from the device tree, but this sram isn't static (it's
a reconfiguration of L2 cache into SRAM mode) and thus can't be described by
mmio-sram.

-Scott