On Wed, Mar 15, 2017 at 4:14 AM, Jonathan Cameron <jic23@xxxxxxxxxxxxxxxxxxxxx> wrote: > > > On 14 March 2017 17:36:16 GMT+00:00, Gargi Sharma <gs051095@xxxxxxxxx> wrote: >>On Tue, Mar 14, 2017 at 2:24 AM, Lars-Peter Clausen <lars@xxxxxxxxxx> >>wrote: >>> On 03/13/2017 09:51 PM, Jonathan Cameron wrote: >>> [...] >>>>>> Gargi, >>>>>> >>>>>> Please insert the new lock above the __cacheline_aligned struct >>>>>> member. >>>>> >>>>> I will do that, but is there any reason why the lock should be >>above >>>>> ____cacheline_aligned struct member? >>>> Yes. It's in fact very important that nothing comes after that. >>>> >>>> Will leave the why as an exercise for the reader. I'll give the >>>> hit of spi drivers that do DMA... >>> >>> >>> One hint though, the answer is somewhere in Documentation/DMA-API.txt >> > >From what I have understood is that cacheline is used for keeping most >>frequently accessed values in adjacent cachelines. > > Why? What does cache line actually mean? > >> We want the >>cacheline_aligned at the end in the struct definition so as to avoid >>any holes after the cacheline boundary. > > Nope. Try > http://www.pebblebay.com/a-guide-to-using-direct-memory-access-in-embedded-systems-part-two/ > > (Hint, search for 'shares') Okay, I got two things from the article: 1. Hardware that uses DMA reads faster if the data is aligned at 16/32/64B boundaries. 2. You want to align buffer being accessed for DMa transfers to prevent cache incoherence. What I still don't understand is why nothing should come after __cacheline_aligned struct member? Thanks, Gargi > >>> >>-- >>To unsubscribe from this list: send the line "unsubscribe linux-iio" in >>the body of a message to majordomo@xxxxxxxxxxxxxxx >>More majordomo info at http://vger.kernel.org/majordomo-info.html > > -- > Sent from my Android device with K-9 Mail. Please excuse my brevity. -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
