Is it safe to use these Linux function (test_bit(), set_bit(), clear_bit()) in character device driver for 2.6.10 ppc kernel.

Misbah khan misbah_khan at
Thu Oct 4 23:02:53 EST 2007

Hi ...
I did followed you and it worked as well. I really Thank you for it. 

At one Place i am doing memcpy() of floating point data to the memory mapped
registers, what could be the substitute of it like "memcpy_toio() " which is
suggested in the Book. I am working BE architecture.

I would really appreaciate if you would let me know the defineation of these
wrapper functions (in_be32,out_be32(),ioread32(),iowrite32(),etc) so that i
could have the clear idea of the reason for not directly dreferencing the


Olof Johansson-2 wrote:
> On Sun, Sep 30, 2007 at 10:38:32PM -0700, Misbah khan wrote:
>> Olof Johansson-2 wrote:
>> > 
>> > First, PLEASE stop quoting your own text. Do not append > in front of
>> > the lines you write yourself in the reply. It makes it impossible to
>> > tell what parts are new and what are old.
> Please read the above again, since you didn't fix your mailer.
> Also, make sure it doesn't prepend spaces in front of the lines you are
> writing.
>> > I am confused that some people tells me to map the memory noncacheble
>> and
>> > some tells me not. could you tell me which is the best approach and
>> please
>> > elaborate the reason as well. The part of the code is mentioned above
>> is a
>> > reference and my concern are as follows:-
> It depends on your application and how the FPGA is attached. Buf if it is
> attached outside of the coherence domain (for example on PCI), then you
> should map it uncacheable. Otherwise you will have to do manual flushing
> of caches to make sure writes make it out to the device, and also flush
> any copy in cache before you read any register. In other words, it makes
> things considerably more complicated and error-prone.
>> > 1. I am mapping 32 KB of memory for which i am using _nocasheble. Is it
>> > absolutely fine????
> Just use ioremap().
>> > 2. I am directly dereferencing the pointer to the mapped region insted
>> of
>> > using a wrapper function due to (1) Aready used in the past and have
>> faith
>> > in it .
> I don't care if you have faith in it or not, it's still not the correct
> way to do it. It might work right some of the time by pure luck but it
> is the incorrect way of accessing device memory.
>> >     (2) I had used functions like ioread32() iowrite32() in the past
>> which
>> > is suggested by rubini in his book on Linux device Driver but the
>> output i
>> > got was bitswapped .
> I assume you mean byte swapped and not bit swapped.
> Are your registers on the device big- or little endian?
> If they are big endian, use in_be32/out_be32. If they're little endian,
> use
> in_le32/out_le32. That will take care of any swapping for you.
>> > 3. test_bit()/clear_bit() are the functions which i am using in my
>> driver
>> > and in the way i described above , please let me know that is looks
>> fine
>> > in the Implimention
> No it is not fine. You cannot use set_bit/clear_bit against noncacheable
> memory. EVER.
>> >  or shall i read the value and mask the bits rather
>> > than beliving in these functions for eg :-
>> >
>> > dfr_data_ret=*(volatile UINT32 *)((volatile UINT32
>> > *)mmap_reg_ptr+DATA_STATUS_REG);
>> > 
>> > dfr_data_ret&=STATUS_MASK;
>> > 
>> > Please reply to clear my doubts.
> Just do what I told you earlier:
> To read the status register, mask out the STATUS_MASK and write it
> back, do:
> val = in_be32(mmap_reg_ptr + DATA_STATUS_REG);
> val &= STATUS_MASK;
> out_be32(MMAP_REG_PTR + DATA_STATUS_REG, val);
> -Olof
> _______________________________________________
> Linuxppc-embedded mailing list
> Linuxppc-embedded at

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