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([2a01:e0a:e17:9700:16d2:7456:6634:9626]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4390d93369fsm184114345e9.3.2025.02.10.06.20.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 10 Feb 2025 06:20:37 -0800 (PST) Message-ID: Date: Mon, 10 Feb 2025 15:20:34 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 7/9] riscv: Prepare for unaligned access type table lookups To: Andrew Jones Cc: Anup Patel , Charlie Jenkins , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, paul.walmsley@sifive.com, palmer@dabbelt.com, jesse@rivosinc.com, Anup Patel References: <20250207161939.46139-11-ajones@ventanamicro.com> <20250207161939.46139-18-ajones@ventanamicro.com> <20250210-e6a2dfcd7995ffc8a6d918e4@orel> Content-Language: en-US From: =?UTF-8?B?Q2zDqW1lbnQgTMOpZ2Vy?= In-Reply-To: <20250210-e6a2dfcd7995ffc8a6d918e4@orel> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 10/02/2025 15:06, Andrew Jones wrote: > On Mon, Feb 10, 2025 at 12:07:40PM +0100, Clément Léger wrote: >> >> >> On 10/02/2025 11:16, Anup Patel wrote: >>> On Sat, Feb 8, 2025 at 6:53 AM Charlie Jenkins wrote: >>>> >>>> On Fri, Feb 07, 2025 at 05:19:47PM +0100, Andrew Jones wrote: >>>>> Probing unaligned accesses on boot is time consuming. Provide a >>>>> function which will be used to look up the access type in a table >>>>> by id registers. Vendors which provide table entries can then skip >>>>> the probing. >>>> >>>> The access checker in my experience is only time consuming on slow >>>> hardware. Hardware that supports fast unaligned accesses isn't really >>>> impacted by this? Avoiding a list of hardware that has slow/fast >>>> unaligned accesses in the kernel was the main reason for dynamically >>>> checking. We did introduce the config option to compile the kernel with >>>> assumed slow/fast accesses, which of course has the downside of >>>> recompiling the kernel and I assume that you already considered that. >>> >>> The kconfig option does not align with the vision of running the same >>> kernel image across platforms. >> >> I'd would be advocating to remove compile time options as well and use >> another way to skip the probe (see below). >> >>> >>>> >>>> Instead of having a table in the kernel, something that would be more >>>> platform agnostic would be to have an extension that signals this >>>> information. That seems like it would accomplish the same goal and >>>> leverage the existing infrastructure in the kernel, albeit with the need >>>> to make a new extension. >>>> >>> >>> IMO, expecting an ISA extension to be defined for all possible >>> microarchitectural choices is not going to scale so it is better >>> to have infrastructure in kernel itself to infer microarchitectural >>> choices based on RISC-V implementation ID. >> >> Since adding an extension seems quite unlikely, and that a device-tree >> property is likely DT centric and not applicable to ACPI as well, was a >> command line argument considered ? >> > > I did consider adding a command line option in addition to the table, > allowing platforms which neither have a table entry [yet] nor want to do > the speed test, to set whatever they like. In the end, I dropped it, since > I don't have a use case at this time. However, if we really don't want a > table, then I can look into the command line option instead. Sorry if I wasn't clear, I wasn't considering this as a replacement for your table but rather as a replacement to Charlie's compile time define to skip misaligned speed probing since it is like "lpj=". You can specify it on command line if you want to skip the loop time detection of loops per jiffies and have faster boot. Regarding your table, it feels like a bit going back to old hardcoded platform description ;). I think some kind of auto-detection of speed (not builtin the kernel) for platforms could be good as well to skip probing. A DT property also seems ok to me since the goal is to describe hardware. Would a common DT/ACPI property be appropriate ? The device_property API unified both so if we used some common property to describe the misaligned access speed (both in DT cpu node/ ACPI CPU device package), we could keep a single parsing method. But I'm no ACPI expert so I don't know if that really make sense. Thanks, Clément > > Thanks, > drew