linux-kernel - Re: [RFC PATCH 1/2] mtd: spi-nor: Introduce bounce buffer to handle vmalloc'd buffers

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Message-ID: <20170301131232.1a3fd2d1@bbrezillon>
Date:   Wed, 1 Mar 2017 13:12:32 +0100
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Frode Isaksen <fisaksen@...libre.com>
Cc:     Cyrille Pitchen <cyrille.pitchen@...el.com>,
        Richard Weinberger <richard@....at>,
        Vignesh R <vigneshr@...com>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Marek Vasut <marek.vasut@...il.com>,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-omap@...r.kernel.org, linux-spi@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] mtd: spi-nor: Introduce bounce buffer to handle
 vmalloc'd buffers

On Wed, 1 Mar 2017 12:18:30 +0100
Frode Isaksen <fisaksen@...libre.com> wrote:

> On 01/03/2017 11:18, Boris Brezillon wrote:
> > On Wed, 1 Mar 2017 11:09:57 +0100
> > Cyrille Pitchen <cyrille.pitchen@...el.com> wrote:
> >  
> >> Le 28/02/2017 à 22:39, Richard Weinberger a écrit :  
> >>> Vignesh,
> >>>
> >>> Am 27.02.2017 um 13:08 schrieb Vignesh R:    
> >>>> Filesystems like UBIFS may pass vmalloc'd buffers to SPI NOR layer which
> >>>> will end up in SPI layer. SPI core does try to handle such buffers (see
> >>>> spi_map_buf()) by doing vmalloc_to_page() and creating scatterlist. But,
> >>>> its known that this does not work well with VIVT/aliasing cache
> >>>> architectures.
> >>>> This also fails when buffers are addressed using LPAE (buffers in region
> >>>> higher than 32 bit addressable region), if DMA is 32bit only.
> >>>>
> >>>> Introduce bounce buffers support in SPI NOR framework to handle
> >>>> vmalloc'd buffers. Use a pre-allocated per flash bounce buffer equal to
> >>>> the sector size of the flash. Flash drivers can enable this feature by
> >>>> setting SNOR_F_USE_BOUNCE_BUFFER flag.
> >>>> This would also enable SPI NOR drivers to safely use DMA in their
> >>>> read/write callbacks.
> >>>>
> >>>> Signed-off-by: Vignesh R <vigneshr@...com>
> >>>> ---
> >>>>  drivers/mtd/spi-nor/spi-nor.c | 30 +++++++++++++++++++++++++++---
> >>>>  include/linux/mtd/spi-nor.h   |  4 ++++
> >>>>  2 files changed, 31 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
> >>>> index 747645c74134..c241fefa5aff 100644
> >>>> --- a/drivers/mtd/spi-nor/spi-nor.c
> >>>> +++ b/drivers/mtd/spi-nor/spi-nor.c
> >>>> @@ -17,6 +17,7 @@
> >>>>  #include <linux/mutex.h>
> >>>>  #include <linux/math64.h>
> >>>>  #include <linux/sizes.h>
> >>>> +#include <linux/mm.h>
> >>>>  
> >>>>  #include <linux/mtd/mtd.h>
> >>>>  #include <linux/of_platform.h>
> >>>> @@ -1205,11 +1206,21 @@ static int spi_nor_read(struct mtd_info *mtd, loff_t from, size_t len,
> >>>>  
> >>>>  	while (len) {
> >>>>  		loff_t addr = from;
> >>>> +		bool use_bb = false;
> >>>> +		u_char *dst_buf = buf;
> >>>> +		size_t buf_len = len;
> >>>>  
> >>>>  		if (nor->flags & SNOR_F_S3AN_ADDR_DEFAULT)
> >>>>  			addr = spi_nor_s3an_addr_convert(nor, addr);
> >>>>  
> >>>> -		ret = nor->read(nor, addr, len, buf);
> >>>> +		if (!virt_addr_valid(buf) && nor->bounce_buf) {    
> >> Should we use is_vmalloc_addr() instead of virt_addr_valid() ?
> >>
> >> I guess virt_addr_valid() returns true even for kmalloc'ed buffers
> >> however the copy into the bounce buffer should be avoided for kmalloc'ed
> >> memory.  
> > The test is !virt_addr_valid(), so we won't use the bounce buffer for
> > kmalloc-ed regions. I don't remember why we use virt_addr_valid()
> > instead of is_vmalloc_addr() in the NAND framework, but there was a
> > good reason (virt_addr_valid() is more restrictive, but I don't
> > remember why it's safer :))  
> I think virt_addr_valid() picks up both kmap'ed and vmalloc'ed pages as not valid...

Thanks for the remainder. So we definitely want to use
virt_addr_valid() here.