Hi Vineet, On Fri, 2018-01-19 at 11:17 -0800, Vineet Gupta wrote: > On 12/18/2017 07:29 AM, Alexey Brodkin wrote: > > If software that was executed before Linux kernel [like boot-ROM or > > bootloader] enabled IOC but we'd like to not use it [mostly for > > debugging of weird DMA issues] we essentially need to disable IOC. > > So we do here. > > > > Note we will only disable IOC if "ioc_enable" variable is force set to 0. > > As of today that's only possible either before building right in > > arch/arc/mm/cache.c or via debugger on target reght before execution of > > the kernel starts. > > > > We may make "ioc_enable" a boot-parameter later though. > > > > Signed-off-by: Alexey Brodkin <abrodkin at synopsys.com> > > --- > > > > Chnages in v2: > > * Now disabling really works as in v1 we never entered > > disabling code if ioc_enable=0. > > * Do nothing if IOC was not enabled > > > > arch/arc/mm/cache.c | 42 ++++++++++++++++++++++++++++++++++++++++-- > > 1 file changed, 40 insertions(+), 2 deletions(-) > > > > diff --git a/arch/arc/mm/cache.c b/arch/arc/mm/cache.c > > index 69f77c113875..9cadf7b779ed 100644 > > --- a/arch/arc/mm/cache.c > > +++ b/arch/arc/mm/cache.c > > @@ -1186,6 +1186,40 @@ noinline void __init arc_ioc_setup(void) > > __dc_enable(); > > } > > > > +/* > > + * Disabling of IOC is quite a tricky action because > > + * nobody knows what happens if there're IOC-ahndled tarnsactions in flight > > + * when we're disabling IOC. > > + * > > + * And the problem is external DMA masters [that were initialized and set in a > > + * bootlaoder that was executed before we got here] might continue to send data > > + * to memory even at this point and we have no way to prevent that. > > Isn't this a sufficient red flag to warrant NOT adding this to kernel ! > > I agree that the use case is bring up of a new RTL version etc with bootloader > already in there - but in that case can we NOT bypass those and run the kernel > bare metal ? > > > + * > > + * That said it's much safer to not enable IOC at all anywhere before > > + * in boot-ROM, bootloader etc but if we do need to disable it in Linux kernel > > + * it should be done as early as possible and made by master core while all > > + * slaves aren't active. > > + * > > Again this is like saying - this is Pandora's Box - don't open it - which is more > of a invitation for others to actually open it ! I agree with all that above. In fact I already sent a pull-request to Tom Rini for a series among other things disabling IOC in U-Boot, see: 1) Pull-request: https://lists.denx.de/pipermail/u-boot/2018-January/317602.html 2) Actual patch: http://git.denx.de/?p=u-boot/u-boot-arc.git;a=commit;h=b0146f9e29ca2e82262416aca65395c322a618f9 So indeed this patch for Linux could be dropped. -Alexey
