RAM bank with features

[Sergey Mironov]

2011/1/28 Russell King - ARM Linux <linux at arm.linux.org.uk>:
> On Fri, Jan 28, 2011 at 05:28:29PM +0300, Sergey Mironov wrote:
>> Hello, community. I am porting Linux to arm1176-based SoC. Our system
>> has 2 RAM banks. First one (EM0) is quite common, but another (EM1)
>> has some "features" :
>> 1) Critical one. There are devices that can't use DMA buffers outside EM1.
>> 2) CPU can read and write to it, but can't execute instructions.
>>
>> What is the finest way of implementing those restrictions? I thought
>> about defining custom memory zone, but I can't see how to link zones
>> with platform_device.dev.dma_mask.
>
> Sigh.  When will hardware people get the idea that adding silly
> restrictions just makes things impossible.
>
> 1. You can't restrict the system such that code only executes from EM0
>   and still use EM1 in the general memory pool.
>
> 2. You can only restrict DMA buffers from physical address zero to a
>   limit determined by a bitmask in Linux.  If this means EM1 is at
>   a lower physical address, you lose.
>
> I think your best solution is going to be to totally ignore EM1, and tell
> the hardware people that they're being silly.  It's really not worth the
> hastle.
>
>> Also, is it possible to allocate user pages from EM1 but prevent users
>> from executing code from them? Say, tell allocator to mark them with
>> L_PTE_* flag?
>
> How would userspace programs execute their code if their pages were
> allocated from EM1?
>

Thanks for answer. Yes, ignoring EM1 was my very first attempt, but
DMA restrictions force me to use this bank. I just have to do it to
work with some devices. Fortunately 1) those devices are all custom
(that is I am the person who is responsible for their drivers and thus
I probably can make static mappings from EM1 to virtual mem range and
pass addresses with platform_data).  2) EM1 has higher addresses than
EM0, so I may try bitmask solution too. What I told about custom zones
is bad idea as I think now.

Read-write-only user pages in EM1 - well, looks like it is truly impossible..

Thanks again!
Sergey