[AMD Official Use Only - General] > -----Original Message----- > From: Krzysztof Adamski <krzysztof.adamski@xxxxxxxxx> > Sent: Friday, July 1, 2022 12:32 PM > To: Datta, Shubhrajyoti <shubhrajyoti.datta@xxxxxxx>; Marek Vasut > <marex@xxxxxxx>; Raviteja Narayanam <raviteja.narayanam@xxxxxxxxxx>; > linux-i2c@xxxxxxxxxxxxxxx; michal.simek@xxxxxxxxxx > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > git@xxxxxxxxxx; joe@xxxxxxxxxxx > Subject: Re: [PATCH v2 03/10] i2c: xiic: Switch to Xiic standard mode for i2c- > read > > [CAUTION: External Email] > > W dniu 30.06.2022 o 10:23, Datta, Shubhrajyoti pisze: > > [AMD Official Use Only - General] > > > > Hi Krzysztof, > > > >> -----Original Message----- > >> From: Krzysztof Adamski <krzysztof.adamski@xxxxxxxxx> > >> Sent: Wednesday, June 29, 2022 7:40 PM > >> To: Marek Vasut <marex@xxxxxxx>; Raviteja Narayanam > >> <raviteja.narayanam@xxxxxxxxxx>; linux-i2c@xxxxxxxxxxxxxxx; > >> michal.simek@xxxxxxxxxx > >> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; > >> linux-kernel@xxxxxxxxxxxxxxx; git@xxxxxxxxxx; joe@xxxxxxxxxxx > >> Subject: Re: [PATCH v2 03/10] i2c: xiic: Switch to Xiic standard mode > >> for i2c- read > >> > >> [CAUTION: External Email] > >> > >> CAUTION: This message has originated from an External Source. Please > >> use proper judgment and caution when opening attachments, clicking > >> links, or responding to this email. > >> > >> > >> Hi Marek, > >> > >> W dniu 29.06.2022 o 16:05, Marek Vasut pisze: > >>>> [...] > >>>> > >>>> If those two modes only differ in software complexity but we are > >>>> not able to support only the simpler one and we have support for > >>>> the more complicated (standard mode) anyways, we know that > standard > >>>> mode can handle or the cases while dynamic mode cannot, we also > >>>> know that dynamic mode is broken on some versions of the core, why > >>>> do we actually keep support for dynamic mode? > >>> If I recall it right, the dynamic mode was supposed to handle > >>> transfers longer than 255 Bytes, which the core cannot do in > >>> Standard mode. It is needed e.g. by Atmel MXT touch controller. I > >>> spent a lot of time debugging the race conditions in the XIIC, which > >>> I ultimately fixed (the patches are upstream), but the long > >>> transfers I rather fixed in the MXT driver instead. > >>> > >>> I also recall there was supposed to be some update for the XIIC core > >>> coming with newer vivado, but I might be wrong about that. > >> It seems to be the other way around - dynamic mode is limited to 255 > >> bytes - when you trigger dynamic mode you first write the address of > >> the slave to the FIFO, then you write the length as one byte so you > >> can't request more than 255 bytes. So *standard* mode is used for > >> those messages. In other words - dynamic mode is the one that is more > >> limited > >> - everything that you can do in dynamic mode you can also do in > >> standard mode. So why don't we use standard mode always for > everything? > > However the current mode is dynamic mode so for less than 255 we can > > use dynamic mode.(the current behavior will not change) Also the > > dynamic mode is nicer on the processor resources. We set the bytes and > the controller takes care of transferring. > > > > However do not have any strong views open to suggestions. > > All I'm saying is that before this patchset, the dynamic mode was used in all > cases and it made sense - it is easier to work with. But it turned out it has its > limitations and support for standard mode was added with several cases that > switch to that mode. The commit message suggests that the only difference is > in how complicated the code for handling them is, does not say why dynamic > mode might still be preferred. And supporting both of them complicates the > code noticeably. > My understanding now is that the code struggles to use the dynamic mode in > all cases that it can because that produces less interrupts and so it is slightly > lighter on resources. So it is a code complication vs effectiveness tradeoff. > Since this is I2C - a slow bus, I'm not sure it is worth it but also don't have > strong opinion on that. If nothing else, I think it would make sense to update > the commit message a little bit to better explain why it is worth keeping both > modes. Will update the commit message in the next version. > > Krzysztof
