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[199.106.103.254]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-2b05fe5653esm93661329eec.1.2026.01.02.15.01.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 02 Jan 2026 15:01:11 -0800 (PST) Message-ID: Date: Fri, 2 Jan 2026 15:01:10 -0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] of: reserved_mem: Allow reserved_mem framework detect "cma=" kernel param To: Rob Herring , Marek Szyprowski Cc: ye.li@oss.nxp.com, kernel@oss.qualcomm.com, saravanak@google.com, akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, robin.murphy@arm.com, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, iommu@lists.linux.dev, quic_c_gdjako@quicinc.com References: <20251210002027.1171519-1-oreoluwa.babatunde@oss.qualcomm.com> <99dc91c9-59fd-47c5-b1d9-157bda86ad59@samsung.com> Content-Language: en-US From: Oreoluwa Babatunde In-Reply-To: Content-Type: text/plain; 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Hence, give >>>> the reserved_mem framework the ability to detect this so that the DT >>>> defined cma region can skip initialization accordingly. >>> Please explain here why this is a new problem. Presumably the >>> RESERVEDMEM_OF_DECLARE hook after commit xxxx gets called before the >>> early_param hook. And why is it now earlier? >>> >>> I don't really like the state/ordering having to be worried about in 2 places. >> >> I also don't like this spaghetti, but it originates from >> commit 8a6e02d0c00e ("of: reserved_mem: Restructure how the reserved >> memory regions are processed") and the first fixup for it: 2c223f7239f3 >> ("of: reserved_mem: Restructure call site for >> dma_contiguous_early_fixup()"). > > Honestly, this code wasn't great before. Every time it is touched it > breaks someone. > >> It looks that it is really hard to make reserved memory >> initialization fully dynamic assuming that the cma related fixups have >> to be known before populating kernel memory pages tables. I also advised >> in >> https://lore.kernel.org/all/be70bdc4-bddd-4afe-8574-7e0889fd381c@samsung.com/ >> to simply increase the size of the static table to make it large enough for the sane use cases, but >> it turned out that this approach was already discussed and rejected: >> https://lore.kernel.org/all/1650488954-26662-1-git-send-email-quic_pdaly@quicinc.com/ > > I guess the question is what's a sane limit? After 128, are we going > to accept 256? I really suspect we are just enabling some further > abuse of /reserved-memory downstream. For example, I could imagine > there's micromanaging the location of media/graphics buffers so they > end up in specific DRAM banks to optimize accesses. No one ever wants > to detail why they want/need more regions. An earlier patch which requested an increase to the static size of the reserved_mem array did include some breakdown as to why a larger size could be needed. Eg: cma regions, dma-buf heaps, Guest VMs, hypervisors, etc. https://lore.kernel.org/all/1650488954-26662-1-git-send-email-quic_pdaly@quicinc.com/ I also see the same problem of if we are using a static size and just increase it to 128, what happens when someone else needs 256? This is why some form of dynamic sizing makes sense to me. > >> Maybe it would make sense to revert the mentioned changes and get back >> to such simple approach - to make the size of the static table >> configurable in the Kconfig? > > I'd rather not resort to a kconfig option. > What issues do you see with using a Kconfig as a solution for this? Regards, Oreoluwa