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[79.22.100.164]) by smtp.gmail.com with ESMTPSA id x11sm327862edq.58.2021.08.28.11.01.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 28 Aug 2021 11:01:50 -0700 (PDT) From: "Fabio M. De Francesco" To: Johan Hovold , Alex Elder , Greg Kroah-Hartman , greybus-dev@lists.linaro.org, linux-staging@lists.linux.dev, linux-kernel@vger.kernel.org, Alex Elder Subject: Re: [greybus-dev] [PATCH v3] staging: greybus: Convert uart.c from IDR to XArray Date: Sat, 28 Aug 2021 20:01:48 +0200 Message-ID: <3554184.2JXonMZcNW@localhost.localdomain> In-Reply-To: References: <20210816195000.736-1-fmdefrancesco@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="utf-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Saturday, August 28, 2021 5:43:49 PM CEST Alex Elder wrote: > On 8/16/21 2:50 PM, Fabio M. De Francesco wrote: > > Convert greybus/uart.c from IDR to XArray. The abstract data type XArray > > is more memory-efficient, parallelisable, and cache friendly. It takes > > advantage of RCU to perform lookups without locking. Furthermore, IDR is > > deprecated because XArray has a better (cleaner and more consistent) API. > > > > Signed-off-by: Fabio M. De Francesco > > I have one more comment, below. Generally, I don't think it is > important to make this change, but I think it's fine to switch > to the newer XArray interface. The result is a little simpler. I agree that the result of using XArray is a little simpler and readable. As far as performance is regarded (memory-efficiency, cache friendliness, parallelization improvements) I have to take for true the words of Matthew W.. Some time ago I did a similar conversion for staging/unisys/visorhba after discussing with him on IRC; he confirmed that the driver would have got several benefits. This is why I decided to do this work on staging/greybus too. I cannot affirm the same for IDA to XArray conversions, since IDA are relatively lighter and efficient than IDR. Unfortunately, I cannot profile such conversions in order to prove/disprove they *really* gain on execution time and/or memory footprint. > > > > [] > > > > static int gb_uart_receive_data_handler(struct gb_operation *op) > > { > > @@ -341,8 +341,8 @@ static struct gb_tty *get_gb_by_minor(unsigned int minor) > > { > > struct gb_tty *gb_tty; > > > > - mutex_lock(&table_lock); > > - gb_tty = idr_find(&tty_minors, minor); > > + xa_lock(&tty_minors); > > I'm basically new to using the XArray interface, but I > don't think you really need the xa_lock()/xa_unlock() > calls here. You are not relying on reference counting > to control when the allocated minor device numbers are > freed, so I'm pretty sure you can simply call xa_load() > to look up the gb_tty for the given minor device. I haven't yet had time to understand how driver works. However, I can attempt a response mostly due to logic than to a real knowledge of how drivers work... (1) I see that alloc_minor is called at "probe" (that I suppose it means the the kernel "feels" that a new device has been added and so it should initialize it somehow and make it ready to operate properly - I hope I'm not too far from the truth :)). (2) I see that xa_alloc() finds an *unused* identifier and, if it succeeds, that identifier is used as the "minor". So, we have one minor per device and that the same minor cannot be re-assigned to other devices. It also should mean that there's no need for reference counting because that "minor" is not shared. (3) If the logic above is sound, we have a 1:1 correspondence between minors and devices (max 16 gb_tty's) and therefore we don't need to lock tty_minors because concurrent code passes different minors to xa_load() which always returns different gb_tty's. If the above argument is wrong I think I should read a book on device drivers for the first time. I have Greg's but I haven't yet opened it for reading :) Thanks, Fabio > But please don't only take my word for it; investigate > it for yourself, and if needed ask others about it so > you're confident it's correct. There is no harm in > taking the lock, but if it's not needed, it would be > nice to avoid it. > > If you conclude the locks are necessary, just say so, > and explain why, and I'll probably just accept it. > Otherwise, please explain why you are sure they are > not needed when you send version 4. Thank you. > > -Alex > > > > + gb_tty = xa_load(&tty_minors, minor); > > if (gb_tty) { > > mutex_lock(&gb_tty->mutex); > > if (gb_tty->disconnected) { > > @@ -353,19 +353,19 @@ static struct gb_tty *get_gb_by_minor(unsigned int minor) > > mutex_unlock(&gb_tty->mutex); > > } > > } > > - mutex_unlock(&table_lock); > > + xa_unlock(&tty_minors); > > return gb_tty; > > }