Re: [Qemu-devel] [PATCH v2] char: cadence: check baud rate generator and divider values

[P J P <ppandit@redhat.com>]

   Hello Alistair,

+-- On Tue, 25 Oct 2016, Alistair Francis wrote --+
| >   * Device model for Cadence UART
| > + *  -> http://www.xilinx.com/support/documentation/user_guides/ug585-Zynq-7000-TRM.pdf
| 
| Can you say what page/section the UART spec is in the Xilinx TRM?

  Chapter 19 UART Controller, page 585, 19.2.3 Baud Rate Generator.
 
| I think it might also be worth noting that the datasheet is a Xilinx
| datasheet that covers the Cadence UART. Others might be using the IP
| as well and might get confused why you are referring to a Xilinx
| datasheet.

  Right, I'll add above section details in the comment.
 
| > +    case R_BRGR: /* Baud rate generator */
| > +        s->r[offset] = 0x028B; /* default reset value */
| 
| Is this the correct behavior, or should the write just be ignored?
| pg.587 of the TRM doesn't really make this clear, did you find this
| somewhere else?

  True, page 587 does not clearly mention if it should be ignored.
But in Appendix B, Register details for 'Baud_rate_gen_reg0' says

    0: Disables baud_sample
    1: Clock divisor bypass (baud_sample = sel_clk)
    2 - 65535: baud_sample

| > +    case R_BDIV:    /* Baud rate divider */
| > +        s->r[offset] = 0x0F;

  Appendix B, Register details for 'Baud_rate_divider_reg0' says

    0 - 3: ignored
    4 - 255: Baud rate


ie. values 0-3 are ignored. But should we avoid writing 's->r[R_BRGR]' & 
's->r[R_BDIV]' for these values? That would lead to undefined values being 
using in 'uart_parameters_setup()', no?

Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
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