Hello Fabio, I haven't done it because I haven't had time and CONFIG_DMA_CMA has not been enabled in the kernel I was fooling with. Yes I do know how to build the kernel. Thanks for all the help. Once I get it working I'll let you know. Cheerio S On Sat, Jan 8, 2022, 17:14 FMDF wrote: > On Sat, 8 Jan 2022, 14:37 Sadanand Warrier, > wrote: > >> Hello Fabio >> Thanks for the response. I had already accessed that link that you >> have pasted. Michal Nazarewicz also has a few other points on using >> CMA. >> When I tried to use the API after setting cma=128M on the kernel boot >> command line and booted the kernel, the api returned without >> allocating memory >> when the driver was installed. >> The dmesg shows that 128M was reserved for CMA but dma_alloc_coherent >> failed. >> It seems that default CentOS and perhaps Ubuntu kernels are not >> compiled with CONFIG_DMA_CMA=yes for the dma_alloc_coherent to work. >> I'm not sure if this is a valid assumption. I haven't tried >> recompiling the kernel yet with that option set. >> From what I have managed to eke out we need CONFIG_CMA=yes and >> CONFIG_DMA_CMA=yes too. >> > > If I recall it well, I think they are needed. Why haven't you yet checked? > > I assume that you know how to change your .config, re-build the kernel and > the modules and install everything in the right places. Don't you? > >> > Just grep the ".config" of your distro. It should be placed in /boot/ > with a name like "config-`uname -r`" or in /usr/src/linux/ with its true > name (.config) and see if those options are enabled. Or you may use > directly "make menuconfig" and search with its special keys. Then "make -jN > && sudo make modules_install install" (and hope that installkernel is not > broken like it has been in openSUSE Tumbleweed for weeks). :( > > I'm not an expert about this particular subject, so if they have already > been enabled I cannot help you with more than the above information. That > article in lwn.net is everything I've studied about this subject and I > haven't ever used it in kernel programming. > > Regards, > > Fabio M. De Francesco > > >> Thanks >> S >> >> >> On Sat, 8 Jan 2022 at 07:23, FMDF wrote: >> > >> > On Tue, 21 Dec 2021, 15:36 Sadanand Warrier, >> wrote: >> >> >> >> Hello >> >> Is CMA still supported as described in this document >> >> https://lwn.net/Articles/396707/ ? >> > >> > >> > Yes, it is. >> > >> > Read the following instructions on how to use that feature: >> > >> > "A deep dive into CMA" at >> > https://lwn.net/Articles/486301/ >> > >> > Regards, >> > >> > Fabio M. De Francesco >> > >> >