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    SubjectRe: [PATCH 4.19 29/74] clk: qcom: regmap-mux: fix parent clock lookup
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    On 15/12/2021 12:16, Pavel Machek wrote:
    > Hi!
    >
    >> The function mux_get_parent() uses qcom_find_src_index() to find the
    >> parent clock index, which is incorrect: qcom_find_src_index() uses src
    >> enum for the lookup, while mux_get_parent() should use cfg field (which
    >> corresponds to the register value). Add qcom_find_cfg_index() function
    >> doing this kind of lookup and use it for mux parent lookup.
    >
    > This appears to have problems with error handling.
    >
    >> +++ b/drivers/clk/qcom/clk-regmap-mux.c
    >> @@ -36,7 +36,7 @@ static u8 mux_get_parent(struct clk_hw *
    >> val &= mask;
    >>
    >> if (mux->parent_map)
    >> - return qcom_find_src_index(hw, mux->parent_map, val);
    >> + return qcom_find_cfg_index(hw, mux->parent_map, val);
    >>
    >> return val;
    >> }
    >
    > So this returns u8.
    >
    >> +int qcom_find_cfg_index(struct clk_hw *hw, const struct parent_map *map, u8 cfg)
    >> +{
    >> + int i, num_parents = clk_hw_get_num_parents(hw);
    >> +
    >> + for (i = 0; i < num_parents; i++)
    >> + if (cfg == map[i].cfg)
    >> + return i;
    >> +
    >> + return -ENOENT;
    >> +}
    >
    > In case of error, -ENOENT will be cast to u8 in caller. I don't
    > believe that is correct.

    Unfortunately there is no way to return proper error code from
    clk_ops->get_parent() callback. However returning -ENOENT would
    translate to 254. Then clk_core_get_parent_by_index() would determine
    that there is no such parent and return NULL. A call to clk_set_parent
    would reparent the clock.

    Returning some sensible default (e.g. 0) would be much worse, since then
    the clock subsystem would assume that the clock has correct parent. A
    call to clk_set_parent would always result in ops->set_parent() call,
    reparenting the clock correctly.

    Most probably it would be correct to make ops->get_parent() return int
    instead of u8 (either an index or an -ERROR). However this was out of
    scope for this patch.

    --
    With best wishes
    Dmitry

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