Should we pass amba device peripheral id with device structure or not?

[Linus Walleij]

2010/5/14 Viresh KUMAR <viresh.kumar at st.com>:

> amba_device_register function reads and updates peripheral id from hardware
> registers, whenever we register any amba device. If clock to device is disabled,
> then amba_device_register will not be able to read and update this value.
>
> Thus device registration will fail.
> (...)

In the U300 arch/arm/mach-u300/clock.c we have this code:

/*
 * These are the clocks for cells registered as primecell drivers
 * on the AMBA bus. These must be on during AMBA device registration
 * since the bus probe will attempt to read magic configuration
 * registers for these devices. If they are deactivated these probes
 * will fail.
 *
 *
 * Please note that on emif, both RAM and NAND is connected in dual
 * RAM phones. On single RAM phones, ram is on semi and NAND on emif.
 *
 */
void u300_clock_primecells(void)
{
        clk_enable(&intcon_clk);
        clk_enable(&uart_clk);
#ifdef CONFIG_MACH_U300_BS335
        clk_enable(&uart1_clk);
#endif
        clk_enable(&spi_clk);

        clk_enable(&mmcsd_clk);

}
EXPORT_SYMBOL(u300_clock_primecells);

void u300_unclock_primecells(void)
{

        clk_disable(&intcon_clk);
        clk_disable(&uart_clk);
#ifdef CONFIG_MACH_U300_BS335
        clk_disable(&uart1_clk);
#endif
        clk_disable(&spi_clk);
        clk_disable(&mmcsd_clk);

}
EXPORT_SYMBOL(u300_unclock_primecells);


When we add the primecells we have this piece:

        /* Register the AMBA devices in the AMBA bus abstraction layer */
        u300_clock_primecells();
        for (i = 0; i < ARRAY_SIZE(amba_devs); i++) {
                struct amba_device *d = amba_devs[i];
                amba_device_register(d, &iomem_resource);
        }
        u300_unclock_primecells();

This way the reference counter is still zero and the clocks are off
when the drivers probe, so they need to enable their own clocks
at the latter stage.

It's a bit brutal but it works. (And yes, we shouldn't be exporting the
symbols, no point, will fix it someday.) However it messes with the
internals of your clock implementation indeed.

One more elegant way would be to pass the master clock along
in struct amba_device like we do with IRQs, so the probe code in
drivers/amba/bus.c know how to clock the device before attempting
to read the PrimeCell IDs. I don't know if that's particularly elegant
though.

Yours,
Linus Walleij