Re: [PATCH 10/11] tty: amba-pl011: add support for 32-bit register access

[Peter Hurley <peter@hurleysoftware.com>]

On 11/03/2015 11:19 AM, Russell King - ARM Linux wrote:
> On Tue, Nov 03, 2015 at 08:57:05AM -0600, Timur Tabi wrote:
>> Russell King wrote:
>>> -	return readw(uap->port.membase + pl011_reg_to_offset(uap, reg));
>>> +	void __iomem *addr = uap->port.membase + pl011_reg_to_offset(uap, reg);
>>> +
>>> +	return uap->access_32b ? readl(addr) : readw(addr);
>>
>> Ok, ignore my previous email.  I just noticed this.
>>
>> This version is fine, except that it now performs a runtime check for every
>> I/O access.  Isn't that too much overhead?  access_32b will always be either
>> True or False for the life of the entire SOC.
> 
> It's a balance between a few more branches and killing six registers
> (r0 - r3, ip, lr) on every register access due to a function call.
> If GCC was a reasonable compiler, it could be done without incuring
> any branches, merely by using conditional instructions, but GCC on
> ARM really sucks - it wants to reload the base address, offset and
> access_32b flag on every device access despite the struct being
> marked const.

The compiler reloads are probably due to the barriers in readw()/writew();
I think the relaxed i/o accessor variants avoid the reloads.

Regards,
Peter Hurley