linux-kernel - Re: [PATCH RFC v2 2/5] of: get dma area lower limit

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87cyn1k7yq.fsf@tarshish>
Date: Thu, 25 Jul 2024 14:49:01 +0300
From: Baruch Siach <baruch@...s.co.il>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Christoph Hellwig <hch@....de>,  Marek Szyprowski
 <m.szyprowski@...sung.com>,  Rob Herring <robh@...nel.org>,  Saravana
 Kannan <saravanak@...gle.com>,  Will Deacon <will@...nel.org>,  Robin
 Murphy <robin.murphy@....com>,  iommu@...ts.linux.dev,
  devicetree@...r.kernel.org,  linux-arm-kernel@...ts.infradead.org,
  linux-kernel@...r.kernel.org,  linuxppc-dev@...ts.ozlabs.org,
  linux-s390@...r.kernel.org,  Petr Tesařík
 <petr@...arici.cz>,  Ramon
 Fried <ramon@...reality.ai>,  Elad Nachman <enachman@...vell.com>
Subject: Re: [PATCH RFC v2 2/5] of: get dma area lower limit

Hi Catalin,

On Tue, Jun 18 2024, Catalin Marinas wrote:
> On Tue, Apr 09, 2024 at 09:17:55AM +0300, Baruch Siach wrote:
>> of_dma_get_max_cpu_address() returns the highest CPU address that
>> devices can use for DMA. The implicit assumption is that all CPU
>> addresses below that limit are suitable for DMA. However the
>> 'dma-ranges' property this code uses also encodes a lower limit for DMA
>> that is potentially non zero.
>> 
>> Rename to of_dma_get_cpu_limits(), and extend to retrieve also the lower
>> limit for the same 'dma-ranges' property describing the high limit.
>
> I don't understand the reason for the lower limit. The way the Linux
> zones work is that ZONE_DMA always starts from the start of the RAM. It
> doesn't matter whether it's 0 or not, you'd not allocate below the start
> of RAM anyway. If you have a device that cannot use the bottom of the
> RAM, it is pretty broken and not supported by Linux.

I won't argue with that assertion. My target system RAM happens to start
at that the lower end of devices DMA zone, so I'm fine with skipping
this patch.

Just curious. What is the inherent limitation that prevents Linux from
supporting DMA zone with lower limit above RAM start?

Thanks,
baruch

-- 
                                                     ~. .~   Tk Open Systems
=}------------------------------------------------ooO--U--Ooo------------{=
   - baruch@...s.co.il - tel: +972.52.368.4656, http://www.tkos.co.il -