MMIO classes
Martin Stein
martin.stein at ...1...
Tue Apr 8 14:01:09 CEST 2014
Hi Aditya,
If I understand you right, all you want is to randomly access the
registers by their array index. This is exactly what 'Register_array'
was created for. The driver 'base-hw/src/core/pic/imx31.h' gives a good
example of how to use it for plain registers. There you have an array
that starts at MMIO base + 0x20 and spans over 8 registers of 32 bit
width each:
struct Nipriority : Register_array<0x20, 32, 8, 32>
{
enum { ALL_LOWEST = 0 };
};
Please note that the second template argument doesn't define the width
of the array items (this is the forth argument) but the internal access
width which affect performance only. Iterative access is done in this
driver later as follows:
for (unsigned i = 0; i < Nipriority::ITEMS; i++)
{
write<Nipriority>(Nipriority::ALL_LOWEST, i);
}
As you can see, you can use the enum 'ITEMS' which is defined by every
'Register_array' as loop parameter. You can also use bit-fields inside
of register arrays instead of having plain registers only. The driver
'base-hw/src/core/pic/arm_gic.h' gives a good example for that:
struct Icfgr : Register_array<0xc00, 32, MAX_INTERRUPT_ID + 1, 2>
{
struct Edge_triggered : Bitfield<1, 1> { };
};
...
for (unsigned i=MIN_SPI; i <= _max_interrupt; i++)
{
_distr.write<Distr::Icfgr::Edge_triggered>(0, i);
...
}
So, here you have a register array 'Icfgr' that starts at MMIO base +
0xc00 and spans over MAX_INTERRUPT_ID + 1 registers of 2 bit width each.
The least significant bit of each register is unused, but the second bit
is defined to be the bit field 'Edge_triggered'. As you can see, the way
of accessing 'Edge_triggered' via the register index 'i' is quite
intuitive if you know how to access plain register arrays. I hope this
helps you with your problem. If you have further questions, don't
hesitate to ask ;-)
Cheers,
Martin
On 08.04.2014 13:23, Aditya Kousik wrote:
> Hello Martin,
>
> Truth be told, I didn't want to probe the use of an object myself.
> But, if I want to create an array of registers that can be iteratively
> called, it saves a lot of effort as opposed to manually writing
> register by register. Since write <..> involves explicit
> 'register_name' here. I'm not sure (or I may not know how) if I can
> create an array of Mmio::Register's since declaration to Register
> necessitates an offset and access width.
>
> Normally, my initial attempt was to create an array of addresses that
> pointed to the starting address of the registers. This gives us random
> access and ability to put R/W accesses in a loop. But it is very
> primitive and does not use the access_t concept.
>
> But then, as you've said I looked into Register_array a bit deeper.
> Here is an approach I've thought of.
> Define all registers with offset values. Then, define a
> Register_array, say R_array that has an offset to the first defined
> register, number of items = number of registers. This works given the
> registers in between are all of the same size (obviously). If there
> are N registers defined, then write<R_array>(value, idx) should write
> to the ith register defined, right? Not necessarily.
>
> Consider this.
>
> class Foo : Mmio
> {
> struct reg1 : Register<0x0, 32> { };
> struct reg2: Register<0x4, 32> { };
> struct reg3: Register<0x10, 32> { };
> struct reg4: Register<0x4c, 32> { };
> struct reg_array : Register_array<0x0, 32,32,4> { };
>
> public:
> void write_to_regs()
> {
> write<reg1>(0);
> write<reg4>(0xff);
> }
> };
>
> Will I achieve random access to the registers with the use of the
> register array? No. Unless, I am fundamentally wrong her, in which
> case, I'd be glad to know where I'm going wrong.
>
> Thanks and regards
> Aditya.
>
>
>
> On Tue, Apr 8, 2014 at 4:23 PM, Martin Stein
> <martin.stein at ...1... <mailto:martin.stein at ...1...>>
> wrote:
>
> Hi Aditya,
>
> Nice to hear that you like the MMIO framework :-) Personally, I
> can't see a benefit in constructing an object of a class that does
> nothing else than inheriting from an Mmio::Register. When trying
> to compile your example, my compiler complains that I can't use
> 'ob' in the constant expression 'write<...>', which sounds
> reasonable to me. Why do you think that 'Register_array' isn't
> what you're looking for? It may be helpful if you could explain
> your use case more precisely.
>
> Cheers,
> Martin
>
>
> On 08.04.2014 11:29, Aditya Kousik wrote:
>> Hello,
>>
>> This doubt is more C++ than Genode. The MMIO framework is
>> brilliant for R/W access. The way it goes is that, I create
>> structures and successively inherit Register and Bitfield
>> structures. But would it be wrong if I create an object of the
>> structures and call 'write' and 'read' methods?
>>
>> Say, struct Register_New : Register<0x4a, 32> { };
>>
>> and struct Register_New ob;
>> write<ob>(0x5);
>>
>> The above is just an example. What is normally done is
>> write<Register_New>(0x5).
>> The template in the Mmio class if of Register::Register_base
>> type. But an object to this struct is also Register. So, is there
>> any negative aspect to this?
>>
>> What I am currently trying out is trying to hold an array of
>> Registers that has such objects for some level of automation. And
>> I don't think Register_array is what I'd look for.
>>
>> Thanks for your patient replies
>> Aditya
>>
>>
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