From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Laight Subject: RE: [PATCH RFC 3/5] Add KSZ8795 switch driver Date: Fri, 29 Sep 2017 09:14:26 +0000 Message-ID: <063D6719AE5E284EB5DD2968C1650D6DD0084EDC@AcuExch.aculab.com> References: <93AF473E2DA327428DE3D46B72B1E9FD41121A87@CHN-SV-EXMX02.mchp-main.com> <20170907223625.GW11248@lunn.ch> <93AF473E2DA327428DE3D46B72B1E9FD41124D5A@CHN-SV-EXMX02.mchp-main.com> <20170928193416.GH14940@lunn.ch> Mime-Version: 1.0 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: 8BIT Cc: "muvarov@gmail.com" , "pavel@ucw.cz" , "nathan.leigh.conrad@gmail.com" , "vivien.didelot@savoirfairelinux.com" , "f.fainelli@gmail.com" , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "Woojung.Huh@microchip.com" To: 'Andrew Lunn' , "Tristram.Ha@microchip.com" Return-path: In-Reply-To: <20170928193416.GH14940@lunn.ch> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org From: Andrew Lunn > Sent: 28 September 2017 20:34 ... > > There are 34 counters. In normal case using generic bus I/O or PCI to read them > > is very quick, but the switch is mostly accessed using SPI, or even I2C. As the SPI > > access is very slow. > > How slow is it? The Marvell switches all use MDIO. It is probably a > bit faster than I2C, but it is a lot slower than MMIO or PCI. > > ethtool -S lan0 takes about 25ms. Is the SPI access software bit-banged? Doing that with software delays isn't friendly to the rest of the system. (Hardware guys please note...) One possibility is to rate-limit the stats reading. Then an application cannot completely 'hog' the SPI bandwidth. David